New GSA Bulletin Articles Published Online Ahead of Print
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics studied this
month include the nature and dynamics of China and Tibet and SE Asia; the
architecture and permeability of the Sibillini Mountains, central
Appenines; the Arctic Atlantic Ocean; and the northern U.S. Cordillera. You
can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Formation of Eocene−Miocene felsic magmatic rocks along
N-S−trending Yardoi-Kongbugang mountain ranges in the eastern
Himalaya: New insights into surface uplift and the initiation of
E-W extension in southern Tibet
Jing-Jing Fan; Xiu-Zheng Zhang; Lin Ma; Qiang Wang; Zi-Qi Jiang ...
Large-scale N-S shortening induced by India-Asia convergence caused the
formation of numerous E-W−trending mountain ranges in Tibet. However, the
mechanism(s) of formation of N-S−trending mountain ranges remains elusive.
We report on a felsic magmatic belt located along the N-S−trending
Yardoi-Kongbugang mountain ranges on the flank of the Cona rift in the
eastern Tethyan Himalaya. Zircon and monazite geochronology and whole-rock
geochemistry revealed three epochs of middle- to lower-crustal anatexis
beneath the Cona rift at ca. 47−42 Ma, 35−34 Ma, and 24−14 Ma. The
mid-Eocene and early Oligocene granitoids show adakitic signatures
indicating continuous crustal thickening, while the formation of Miocene
leucogranites and N-S−trending dacitic dikes was related to ductile crustal
extension. Silicic melts were exposed along the whole rift since the early
Oligocene, suggesting that the early Oligocene could be regarded as a
transitional epoch from tectonic compression to orogen-parallel extension.
Widespread mid-Eocene and Miocene magmatism in the Himalaya, together with
coeval metamorphic anatexis, represents two phases of crustal weakening.
The weakened crustal zones under continued India-Asia convergence may have
favored uplift and subsequent lateral flow of the weak zones, which
initiated E-W extension. Finally, significant upwelling of the weak zones
evolved into magma extrusion and formed the N-S−trending mountain ranges.
This study provides new insights into the mechanisms of surface uplift and
E-W extension and challenges the common view of initiation of E-W extension
in southern Tibet not earlier than the early Miocene.
Geochemical signature of a deeply subducted slab in the source of
post-collisional magmatism: A case study from the Western Tianshan
NW China
Ning-Bo Li; He-Cai Niu; Qiang Shan; Wu-Bin Yang
The contributions of deeply subducted slabs to post-collisional magmatism
are poorly constrained. Here, we present a study of the post-collisional,
Early Permian (ca. 279 Ma), Hamisite basalts from the Western Tianshan NW
China to trace the geochemical fingerprints of deeply subducted slab. The
Hamisite basalts have arc-like trace element patterns but high Nb/Ta ratios
(18.4−23.0). They have slightly enriched Sr−Nd isotopic compositions ( 87Sr/86Sr = 0.7048−0.7052; εNd[t]
= 2.02−3.85), which indicates that the basalts were derived from an
enriched mantle source. Extremely low δ7Li values (−5.76‰ to
0.20‰) imply the involvement of a deeply subducted slab (i.e., eclogites)
in the source. Correlations among Nb/Ta and (Dy/Yb)N ratios, and
rare earth element contents, indicate that the deeply subducted slab
contained residual rutile and garnet but no epidote. Given that melting of
eclogites requires an anomalously hot mantle source, we propose that mantle
upwelling during post-collisional extension triggered melting of the deeply
subducted slab. Our results indicate that melting of a deeply subducted
slab can generate a metasomatized mantle source with high Nb/Ta ratios,
which could be a hidden, high-Nb/Ta reservoir on Earth. Low δ7Li
values and high Nb/Ta ratios could be a fingerprint of recycling of a
deeply subducted slab.
A tale of an orbicule in the eastern Gangdese belt of southern
Tibet: Petrographic, geochemical, and submagmatic structural
perspectives on its formation
Xuxuan Ma; Zhiqin Xu; Xiaohong Mao; Tarryn Cawood; Haoyu Yan ...
Owing to their intriguing appearance and unusual occurrence, as well as
their significance in addressing magmatic processes, orbicules have
attracted attention from geologists all over the world. We discovered an
orbicule-bearing boulder in the Linzhi region, near the Eastern Himalayan
Syntaxis, which is the first report of orbicules in the Gangdese magmatic
belt, southern Tibet. The orbicules are composed of dioritic shells around
monzodioritic cores, in a dioritic matrix. They typically have a single
shell, composed of elongate and radially oriented hornblendes, with
interstitial plagioclase. Zircon laser ablation−inductively coupled
plasma−mass spectrometry U-Pb dating yields an indistinguishable age of ca.
28.8 Ma for both the orbicule core and matrix, indicating approximately
coeval crystallization. Zircon Lu-Hf and whole-rock Sr-Nd isotopes reveal
that the original magmas of both the orbicule matrix and orbicule core were
relatively juvenile (ƐHf(t) = +2.2 to +6.0 for matrix and +1.2
to +3.9 for core; ƐNd(t) = −1.68 to −1.46 for matrix and −3.65
to −2.52 for core), similar to reported Oligocene−Miocene post-collisional
adakites in the Gangdese belt. Zircon and hornblende geochemistry reveals
that both the orbicule matrix and cores were generated from highly
oxidized, high oxygen fugacity magmas. Hornblende composition further
indicates that the original magmas of the matrix and the orbicule cores
were water-rich, with H2O contents greater than 4 wt%. In
combination with the whole-rock geochemistry, these results suggest that
the orbicules were formed in a post-collisional setting, from magma
generated by partial melting of the juvenile thickened lower crust of the
Lhasa terrane triggered by convective removal of the Lhasa lithospheric
root. The preferred orientation of euhedral igneous minerals in the matrix
and the alignment and compaction of orbicules define a magmatic to
submagmatic foliation. This, together with the subsolidus deformation
microstructures exhibited by minerals within the orbicules, indicates that
some flow or movement of orbicules took place under magmatic to submagmatic
conditions, as the subsolidus plastic orbicules were jostled and
transported within a hypersolidus liquid magma.
Syn-collisional extension and Ni-Cu sulfide-bearing mafic magma
emplacement along the Irtysh Shear Zone in the Central Asian
Orogenic Belt
Bo Wei; Christina Yan Wang; Pengfei Li
The Central Asian Orogenic Belt was characterized by a long-lived
accretionary history from Neoproterozoic to Paleozoic, followed by a
collisional phase of orogeny in the latest Paleozoic with the closure of
the Paleo-Asian Ocean. However, Ni-Cu sulfide mineralization associated
with mantle-derived mafic magmas in the Central Asian Orogenic Belt was
relatively short-lived. Thus, the coherence between the protracted
accretionary and collisional history of the Central Asian Orogenic Belt and
the transient Ni-Cu sulfide-bearing mafic magmatism is difficult to
understand. We investigated both Ni-Cu sulfide-bearing and sulfide-barren
mafic intrusions along a major suture zone of the Irtysh Shear Zone within
the Central Asian Orogenic Belt to understand the origin of short-lived
Ni-Cu sulfide mineralization in the context of the collision of two arc
systems of the Chinese Altai and the East Junggar regions. Zircon U-Pb
dating results for the Ni-Cu sulfide-bearing mafic intrusions show that
they were only emplaced briefly from 290 Ma to 280 Ma after subduction
terminated (ca. 312 Ma), substantially earlier than Ni-Cu sulfide-barren
mafic intrusions (ca. 274−271 Ma). Zircons from all Ni-Cu
sulfide-bearing/barren mafic intrusions yield arithmetic mean δ 18O values ranging from 6.0 to 7.4‰, higher than the typical
mantle value (∼5.3‰), which indicates their derivation from a metasomatized
mantle with inherited subduction signatures. The δ11B values of
the rocks from these intrusions range from −17.2‰ to −2.3‰, significantly
lower than those of typical volcanic arcs (up to +18‰), which suggests that
the metasomatic components in the mantle source were dominated by sediment
melts rather than fluids. The pressure calculated using amphibole
thermobarometry shows that the parental magmas of the Ni-Cu sulfide-bearing
mafic intrusions were emplaced shallower than those of Ni-Cu sulfide-barren
mafic intrusions. This is supported by the fact that the country rocks of
the Ni-Cu sulfide-bearing mafic intrusions are shallow crustal lithologies
of sedimentary rocks, whereas the country rocks of the barren intrusions
are relatively deeper crustal lithologies of high-grade metamorphic rocks
and granites. Olivine from the Ni-Cu sulfide-barren intrusions has
forsterite (Fo) contents ranging from 51.1 mol% to 76.2 mol% and Ni from
196 ppm to 1312 ppm. These values are much lower than those of the Ni-Cu
sulfide-bearing intrusions (Fo = 72.6−79.4 mol%; Ni = 1022−1925 ppm), which
indicates that the parental magmas of the barren intrusions may have
experienced extensive olivine crystallization. Notably, the emplacement of
the Ni-Cu sulfide-bearing mafic intrusions (290−280 Ma) coincidently
overlapped with a transient period of orogen-parallel extension (ca.
295−280 Ma) in response to the collision of the Chinese Altai and East
Junggar. Syn-collisional extension may lead to the rapid ascent and
emplacement of mantle-derived mafic magmas at a shallower crustal level,
preventing significant loss of Ni from olivine crystallization at depths,
and thus providing metal-rich mafic magmas for potential Ni-Cu sulfide
mineralization. Our results highlight that syn-collisional extension is an
essential geodynamic mechanism that controls the emplacement of Ni-Cu
sulfide-bearing mafic intrusions in orogenic belts.
Architecture and permeability structure of the Sibillini Mts.
Thrust and influence upon recent, extension-related seismicity in
the central Apennines (Italy) through fault-valve behavior
Manuel Curzi; Angelo Cipriani; Luca Aldega; Andrea Billi; Eugenio Carminati
...
The central Apennines are a fold-thrust belt currently affected by
post-orogenic extensional seismicity. To constrain the influence that the
inherited thrust-related structures exert on the present seismic behavior
of the belt, we provide the high-resolution structural and hydraulic
characterization of one of the most external exposed thrust fault systems
of the central Apennines, the Sibillini Mts. Thrust Front (STF). We
integrate structural mapping, multiscale structural analysis, and in situ
air permeability on the brittle structural facies of the thrust zone. We
also performed K-Ar dating of selected fault rocks to better constrain
structural inheritance. The STF is defined by a complex, ∼300-m-thick
deformation zone involving Meso-Cenozoic marl and limestone that results
from the accommodation of both seismic and aseismic slip during shortening.
Permeability measurements indicate that the low permeability (10 −2 ÷ 10−3 D) of the marly rich host rock diminishes
within the thrust zone, where the principal slip surfaces and associated
S-C structures represent efficient hydraulic barriers (permeability down to
∼3 × 10−10 D) to sub-vertical fluid flow. Field data and K-Ar
dating indicate that the STF began its evolution ca. 7 Ma (early
Messinian). We suggest that the studied thrust zone may represent a barrier
for the upward migration of deep fluids at the hypocentral depth of
present-day extensional earthquakes. We also speculate on the influence
that similar deformation zones may have at depth on the overall regional
seismotectonic pattern by causing transient fluid overpressures and,
possibly, triggering cyclic extensional earthquakes on normal faults prone
to slip while crosscutting the earlier thrust zones (as per a classic fault
valve behavior). This mechanism may have controlled the origin of the
2016−2017 central Apennines devastating earthquakes.
Paleomagnetism and geochronology of Early Cretaceous volcanic rocks
in the eastern segment of the Lhasa terrane, Tibetan Plateau, and
their tectonic implications
Yong Cao; Zhiming Sun; Haibing Li; Zhenyu Yang; Junling Pei ...
Paleogeographic reconstructions of the Lhasa terrane for the Cretaceous
provide important constraints on the evolution of the Neo-Tethys Ocean and
crustal shortening within Asia. However, the Cretaceous paleogeography of
the Lhasa terrane remains contentious. A direct way to study this issue is
to conduct paleomagnetic investigations of the Cretaceous rocks of the
Lhasa terrane; however, most previous Cretaceous paleomagnetic
investigations of the Lhasa terrane were conducted in the middle and
western segments of this terrane. Different vertical-axis rotations
affected different parts of the Lhasa terrane following the India-Asia
collision, and therefore paleomagnetic data from the western and middle
segments of the Lhasa terrane cannot necessarily be used to directly
constrain the paleolatitudes of its eastern segment. This study presents
paleomagnetic data from the Early Cretaceous volcanic rocks with
well-constrained ages from the Luolong area in the eastern segment of the
Lhasa terrane. Isotope geochronology reveals that these rocks formed at
127−124 Ma. The mean high-temperature direction obtained from 17
paleomagnetic sites is declination/inclination (Ds/Is) =
21.8°/19.0° with κs = 65.8 and α95 = 4.4° (κ—best estimate of
the precision parameter; α95—radius of the 95% probability
ellipse around the mean direction; s—stratigraphic coordinates).
Petrographic investigations, a positive fold test, a reversal test, and a
paleosecular variation test indicate the primary origin of this
characteristic remanence. A paleomagnetic pole of 60.9°N, 227.2°E with dp/dm = 2.4°/4.6° (dp/dm—semi-axes of the 95% probability ellipse
around the mean pole) yields a paleolatitude of 9.2 ± 2.4°N for the eastern
segment of the Lhasa terrane. Combined with reliable results from previous
paleomagnetic studies, we draw the following conclusions. (1) During the
Early Cretaceous, the Lhasa terrane was oriented WNW-ESE as a whole, and
the eastern-middle segments may have been oriented nearly E-W. (2) Asia has
accommodated 2050 ± 230 km of N-S crustal shortening along 96°E longitude
since the Early Cretaceous. (3) The minimum N-S width of the Neo-Tethys
Ocean at ca. 125 Ma was 4185 ± 300 km.
Tectono-magmatic evolution of the Greenland−Iceland−Norway ridge
complex and the Jan Mayen hotspot in the Arctic Atlantic Ocean:
Constraints from in situ trace elements and Sr isotopes of minerals
Shuang-Shuang Chen; Rui Gao; Jian-Ping Zheng; Jia-Qi Liu
The subarctic North Atlantic Ocean has experienced a complex and gradual
magmatic evolution, including continental breakup, mature seafloor
spreading, and episodic ridge jumps. Here, we present new in situ
major-element, trace-element, and Sr isotopic compositions for minerals
(olivine, clinopyroxene, and plagioclase) from the Jan Mayen microcontinent
(Integrated Ocean Drilling Program [IODP] Sites 907 and 985), Reykjanes
Ridge (Site 983), Greenland Basin (Site 913), and Knipovich Ridge (Site
911) volcanic samples in the Arctic Atlantic Ocean. The Jan Mayen
hotspot−type and Iceland plume−type volcanic rocks have a common magma
source. The former were likely derived from the latter by further
fractional crystallization, resulting in their distinct geochemical
features. The in situ incompatible element and isotopic compositions show
that the normal mid-ocean-ridge basalt spreading ridge feature for the
older (>14 Ma) system and the enriched plume feature for the younger
(1.521 Ma and 1.049 Ma) system likely indicate plume-ridge interaction and
the evolution from mid-ocean-ridge−type to plume-type magmatism beneath the
Norwegian−Greenland Sea. The disequilibrium of Sr isotopes in plagioclase
among the groundmass, phenocryst cores, and rims is likely due to
heterogeneous compositions beneath the North Atlantic Ocean basin, with the
contamination of thick continental crust and the influence of H2
O-enriched melts originating from the deep Iceland plume.
Astronomical and tectonic influences on climate and deposition
revealed through radioisotopic geochronology and Bayesian age-depth
modeling of the early Eocene Green River Formation, Wyoming, USA
Benjamin T. Bruck; Brad S. Singer; Mark D. Schmitz; Alan R. Carroll;
Stephen Meyers ...
The Wilkins Peak Member (WPM) of the Green River Formation in Wyoming, USA,
comprises alternating lacustrine and alluvial strata that preserve a record
of terrestrial climate during the early Eocene climatic optimum. We use a
Bayesian framework to develop age-depth models for three sites, based on
new 40Ar/39Ar sanidine and 206Pb/ 238U zircon ages from seven tuffs. The new models provide two-
to ten-fold increases in temporal resolution compared to previous
radioisotopic age models, confirming eccentricity-scale pacing of WPM
facies, and permitting their direct comparison to astronomical solutions.
Starting at ca. 51 Ma, the median ages for basin-wide flooding surfaces
atop six successive alluvial marker beds coincide with short eccentricity
maxima in the astronomical solutions. These eccentricity maxima have been
associated with hyperthermal events recorded in marine strata during the
early Eocene. WPM strata older than ca. 51 Ma do not exhibit a clear
relationship to the eccentricity solutions, but accumulated 31%−35% more
rapidly, suggesting that the influence of astronomical forcing on
sedimentation was modulated by basin tectonics. Additional high-precision
radioisotopic ages are needed to reduce the uncertainty of the Bayesian
model, but this approach shows promise for unambiguous evaluation of the
phase relationship between alluvial marker beds and theoretical
eccentricity solutions.
Large-scale basement mobilization endows the giant Carlin-type gold
mineralization in the Youjiang Basin, South China: Insights from
mercury isotopes
Wei Gao; Ruizhong Hu; Xueyun Wang; Runsheng Yin; Xianwu Bi ...
The metal source and genesis of hydrothermal mercury-rich gold metallogenic
systems occurring far away from active continental margins remain puzzling.
The Youjiang Basin of South China, where exists numerous Carlin-type gold
deposits and some synmineralization hidden intrusions, is a natural
laboratory to address this issue due to it was up to 1000 km inward from
the active continental margins of South China when mineralization. Here, we
use mass-independent fractionation of mercury isotope ratios (reported as ∆ 199Hg), which is predominantly generated during Hg photochemical
reactions on Earth’s surface and has superiority of isotopic inheritance
during hydrothermal processes, to address the metal source of the Youjiang
Carlin-type gold deposits. Ore-associated sulfides from seven
representative deposits display negative to near-zero ∆199Hg
values (−0.29‰ to 0.04‰), which fall in between that of the regional
Precambrian basement rocks (−0.21‰ to 0.06‰) and deep magmatic-hydrothermal
systems (∼0‰), suggesting a binary mixing of Hg from these two sources. An
isotope mixing model and mass balance calculations demonstrate that ∼1000
km3 of the basement rocks, which contributed to 86% of Hg
budget, were leached and remobilized by magmatic-hydrothermal fluids and
deep-circulating crustal fluids to endow the gold reserves of these
deposits. Given that traditional S, Pb, C, and O isotopic data yielded
indirect and ambiguous constraints on metal source due to their complex
evolution processes and isotope fractionation during the fluids ascended.
Our results, therefore, highlight the great advantage of using Hg isotope
as a new tracer to understand metal sources of hydrothermal deposits.
Late Miocene or older canyon incision in the northern U.S.
Cordillera shown by erosion rates, incision models, and basalt flow
ages
Nate Mitchell; Brian Yanites; Alison Duvall; Eugene Humphreys; Jonathan
Perry-Houts ...
Deep canyons along the Salmon, Snake, and Clearwater rivers in central
Idaho, USA suggest long-lasting transient incision, but the timing and
drivers of this incision are not well understood. The perturbation of the
Yellowstone hotspot, eruption of flood basalts, and drainage of Lake Idaho
all occurred within or near to this region, but the relationship among
these events and incision is unclear. Here, we utilized in situ 10Be cosmogenic radionuclide concentrations for 46 samples (17
new) of fluvial sediment across the region to quantify erosion rates,
calibrate stream power models, and estimate incision timing. We estimate
that transient incision along the Salmon River began prior to ca. 10 Ma.
However, canyon age decreases to ca. 5 Ma or earlier farther to the north.
For a group of tributaries underlain by basalt, we use the age of the
basalt to estimate that local transient incision began between ca. 11.5 and
5 Ma. Based on these timing constraints, the canyons along the Salmon and
Clearwater rivers predate the drainage of Lake Idaho. We argue that canyon
incision was triggered by events related to the Yellowstone hotspot (e.g.,
basalt lava damming, subsidence of the Columbia Basin, reactivation of
faults, and/or lower crustal flow). Furthermore, our models suggest basalt
may be more erodible than the other rock types we study. We show that
lithology has a significant influence on fluvial erosion and assumptions
regarding river incision model parameters significantly influence results.
Finally, this study highlights how geodynamic processes can exert a
significant influence on landscape evolution.
High-magnesium igneous associations record final-stage geodynamic
process of the southeastern segment of the Central Asian Orogenic
Belt
Yan Jing; Wen-chun Ge; Hao Yang; Yu Dong; Zheng Ji ...
Much controversy exists regarding the evolution and termination of
accretionary orogens where typical continent-continent collision features
are lacking. The Central Asian Orogenic Belt provides a key opportunity to
explore the genetic relationships between the formation of high-Mg igneous
associations and the evolution of accretionary orogens. In this study,
two-stage Triassic magmatic events were identified in the southeastern
segment of the Central Asian Orogenic Belt (including ca. 247 Ma andesites
and ca. 222 Ma diorites), through precise secondary ion mass spectrometry
(SIMS) and laser ablation−inductively coupled plasma−mass spectrometry
(LA-ICP-MS) zircon dating. The Olenekian andesites show typical sanukitoid
affinities with high TiO2, MgO, Ni, and Cr contents, and
enrichment in large ion lithophile elements (LILEs; K, Ba, and Sr). They
yield depleted Hf isotopic compositions and high δ18O values,
which along with Hf-O isotope modeling results indicate that they
originated from the partial melting of mantle wedge peridotites with input
of 20% terrigenous sediments. The Norian diorites have moderate SiO 2, relatively high MgO, Cr, and Ni contents, Mg#
values of 63−65, high Sr, low Y and Yb contents, relevant high La/Yb
ratios, and negligible Eu anomalies, which makes them comparable to high-Mg
andesites with adakitic affinity. Their depleted isotopic compositions are
similar to coeval A-type granites that resulted from lithospheric
delamination after the final collision. In addition, with high K 2O/Na2O ratios and incompatible element ratios
similar to those of the continental crust (e.g., Nb/U, Ce/Pb, and Nd/Sm),
they conform to an origin in which partial melting of delaminated mafic
lower crust subsequently interacted with mantle materials. These two-stage
abnormal Triassic high-Mg rocks archive a hot slab window triggered by the
break-off of the Paleo-Asian oceanic slab and lower crustal delamination
related to collapse of the southeastern segment of the Central Asian
Orogenic Belt. These findings provide beneficial information for the
reconstruction of the East Asian blocks in Pangea.
The genesis of ca. 1.78 Ga granitoids in the Xiong’er large igneous
province: Implications for continental crust generation
Jian-Feng Ma; Chuan-Hao Qu; Yan-Yan Zhou; Tai-Ping Zhao
The genesis of intermediate-felsic rocks and the implications for crustal
growth are highly controversial. These issues are particularly relevant to
large igneous provinces, which are one of the most effective methods of
crustal generation. Mafic magmas, even those with continental marks, have
been considered as the contribution from mantle to crust, whereas the
intermediate-felsic series in large igneous provinces has uncertain
implications for crustal growth due to uncertainty about their modes of
generation. In this paper, we focus on the intermediate-felsic granitoids,
i.e., Gushicun diorites and granites, in the Xiong’er large igneous
province (ca. 1780‒1750 Ma) through detailed study of their petrology,
chronology, and geochemistry. Our new data show that both diorites and
granites have the same emplacement age of ca. 1780 Ma and were synchronized
with the Xiong’er large igneous province. The granitoids are characterized
by enrichment in light rare earth elements (LREEs) and large ion lithophile
elements (LILEs; e.g., Li, Ba, and Rb) but depletion in high field strength
elements (HFSEs; e.g., Nb, Ta, and Ti). They have almost the same ΣREE
(rare earth element) contents and TTi-Zircon (∼850 °C),
whole-rock Nd (εNd(t) from −8.24
to −5.76), zircon Hf (εHf(t)
isotope ratios of −14 to −8 isotope ratios), and O-isotope compositions
(average δ18O = 5.3‰). The fine-grained Gushicun diorites have
low Sr/Nd (<10) and Eu/Sm (<0.3) but high Si/Al (>3.2) ratios and
are similar to the basaltic andesites in the Xiong’er large igneous
province, which suggests that the diorites are crystallization melts that
differentiated along the liquid line of descent. The Rayleigh fractionation
model based on the REE contents shows that the Gushicun diorites were
formed by 40%‒50% fractional crystallization of Xiong’er basaltic melts.
The characteristics of high K2O (3.8‒4.7 wt%) and Yb contents
(4.0‒6.5 ppm), low Sr/Y (1.4‒3.3), and low metaluminous values (A/CNK =
0.8‒1.1) of the Gushicun granites with mantle-like O-isotope values rule
out the melting genesis of low K2O (<0.5 wt%) amphibolite and
trondhjemite−tonalite−granodiorites (TTGs) of the surrounding Taihua
Complex. The Gushicun granites show a geochemical affinity with andesites
in the Xiong’er large igneous province. The alphaMELTs simulation based on
the major elements argues against the partial melting of amphibolite and
TTGs but supports the melting genesis of Xiong’er intermediate igneous
rocks with low water contents (<1 wt%). These findings shed light on the
formation of the felsic series in large igneous provinces, which remelted
the previous mafic-intermediate rocks via underplating basaltic magmas.
Despite having enriched Nd- and Hf-isotope features, the Gushicun diorites
represent juvenile crust, while the granites were formed by the reworking
of juvenile crust instead of ancient crust. This paper also suggests that
radiogenic isotopic ratios cannot be reliably used to assess types of
continental crust.
Evolutionary, paleoecological, and biostratigraphic implications of
the Ediacaran-Cambrian interval in West Gondwana
Lucas V. Warren; Lucas Inglez; Shuhai Xiao; Luis A. Buatois; M. Gabriela
Mángano ...
The Ediacaran-Cambrian transition interval is described for the west part
of the Gondwana Supercontinent. This key interval in Earth’s history is
recorded in the upper and lower part of the Tagatiya Guazú and Cerro Curuzu
formations, Itapucumi Group, Paraguay, encompassing a sedimentary
succession deposited in a tidally influenced mixed carbonate-siliciclastic
ramp. The remarkable presence of cosmopolitan Ediacaran shelly fossils and
treptichnids, which are recorded in carbonate and siliciclastic deposits,
respectively, suggests their differential preservation according to
lithology. Their distribution is conditioned by substrate changes that are
related to cyclic sedimentation. The associated positive steady trend of
the δ13C values in the carbonate facies indicates that the
Tagatiya Guazú succession is correlated to the late Ediacaran positive
carbon isotope plateau. Sensitive high-resolution ion microprobe U-Pb ages
of volcanic zircons from an ash bed ∼30 m above the fossil-bearing interval
in the Cerro Curuzu Formation indicate an Early Cambrian (Fortunian)
depositional age of 535.7 ± 5.2 Ma. As in other coeval sedimentary
successions worldwide, the co-occurrence of typical Ediacaran skeletal taxa
and relatively complex trace fossils in the studied strata highlights the
global nature of key evolutionary innovations.
Molybdenum mineralization genetically linked with magmatism at the
Shipingchuan deposit, SE China
Yan-Jun Li; Jun-Hao Wei; Meng-Ting Chen; Zheng-Hui Chen; Yann Lahaye ...
Most molybdenum (Mo) deposits are considered to be associated with magmatic
systems; however, their genetic link is not clearly resolved when using,
for example, bulk sulfur (S) and lead (Pb) isotopes of sulfides dominated
by pyrite separates. Here, we present microtextures and in situ
trace-element results of pyrite, in situ S isotopic compositions of
molybdenite and pyrite, and Pb isotopes of pyrite and K-feldspar determined
by laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS)
and multicollector (MC) LA-ICP-MS. Combined with geochronologic data for
both the magmatism and Mo mineralization, these in situ methods allowed the
genetic link between Mo mineralization and magmatism to be unraveled at
Shipingchuan, SE China. The Shipingchuan deposit is characterized by
molybdenite-quartz veins hosted by faults or fractures around a biotite
K-feldspar granite. Zircon LA-ICP-MS U-Pb and molybdenite Re-Os dating
results confirm that the biotite K-feldspar granite and mineralization were
coeval and formed during the end of the Early Cretaceous (ca. 105 Ma),
whereas postmineralization monzogranite porphyry dikes formed at 93.7 Ma.
Pyrite shows a close mutual relationship with molybdenite and is
characterized by Co-Ni-As-Te-Cu-Pb-Ag-Bi−rich cores (PyI) revealed by
LA-ICP-MS data. Co/Ni and calculated (Se/S)fluid ratios for both
cores (PyI) and rims (PyII) indicate a magmatic-hydrothermal origin. The in
situ S isotopic compositions (δ34S) of molybdenite are 2.1‰ to
6.8‰, which are identical to those of molybdenite separates. PyI has δ 34S values of −6.9‰ to 0.8‰, whereas PyII exhibits slightly more
negative values of −7.5‰ to −0.2‰. These analyses indicate reliable results
for in situ S isotopes in molybdenite, which could be more appropriate to
constrain the origin of sulfur in hydrothermal fluids than results from
pyrite analysis. The in situ S isotopic compositions of sulfides confirm a
magmatic source for sulfur. The in situ Pb isotopic compositions of pyrite
are consistent with those of K-feldspar from the biotite K-feldspar
granite, indicating a common source. The heterogeneous As/Ni, As/Sb, and
As/Bi values, as well as the variations of δ34S values of both
PyI and PyII, indicate progressive oxidization of the hydrothermal fluids.
These results illustrate that the metal-rich fluids were released from the
granite and migrated along faults and fractures around the granite in an
extensional setting. The precipitation of molybdenite-quartz veins at
Shipingchuan was triggered by changes in temperatures and redox conditions,
which were caused by mixing of magmatic fluids and meteoric water. Results
in this study provide an example of tracing the link between Mo
mineralization and magmatism using in situ S isotopic compositions of
molybdenite and Pb isotopes of pyrite and K-feldspar.
Apatite records metamorphic and hydrothermal fluid evolution at the
large Shuangqishan orogenic gold deposit, SE China
Ying Ma; Shao-Yong Jiang; Hartwig E. Frimmel
Orogenic gold deposits are currently the world’s major source of Au, but
uncertainty exists in the timing of mineralization relative to the ages of
host rocks, metamorphism, and magmatism. Consequently, the origin and
detailed evolution of ore-forming fluids have long been elusive. Apatite
occurs widespread in various types of mineral deposits and can provide
valuable information on ore genesis. Here, we present textural,
high-precision in situ U-Pb geochronological, trace elements, and Sr
isotope data for metamorphic and hydrothermal apatite in the Shuangqishan
gold deposit in southeastern China. These data, together with U-Pb dates of
metavolcanic host rocks, granitic and mafic intrusive rocks, allow us to
precisely constrain the timing of mineralization and reconstruct the
history of the ore-forming fluids there. The apatite crystals within
metamorphic and auriferous quartz veins can be grouped into three
generations according to cathodoluminescence imaging and trace element
concentrations: Ap1 precipitated in the pre-mineralization metamorphic
stage, whereas Ap2 and Ap3 formed in the auriferous hydrothermal stage.
Metamorphic Ap1 displays a negative Eu anomaly, indicating a predominance
of Eu2+ and crystallization under relatively reduced conditions.
On the other hand, syn-gold hydrothermal Ap2 and Ap3 have marked positive
Eu anomalies. Radiometric ages for metamorphic Ap1 are at ca. 461 Ma,
whereas hydrothermal Ap2 and xenotime from auriferous quartz gave U-Pb ages
of 425 ± 15 Ma and 416 ± 15 Ma, interpreted as the time of gold
mineralization. Emplacement ages determined by U-Pb zircon data are 439 ± 2
Ma for granitic intrusion and 427 ± 2 Ma for a mafic dike in the mining
district. Mineralization, therefore, postdates the metamorphism of local
host rocks and granitic magmatism by ∼35 ± 24 m.y. and ∼14 ± 17 m.y.,
respectively. The synchronism of gold genesis and mafic magmatism provides
additional evidence for a mantle derivation of the ore-forming fluids. The
highly radiogenic 87Sr/86Sr ratios obtained for Ap1
(0.7178−0.7302) are consistent with its precipitation from a metamorphic
fluid. Higher Sr concentrations (Ap2 = 1228−2884 ppm, Ap3 = 2325−3169 ppm)
and positive Eu anomalies (Eu/Eu*: Ap2 = 1.03−2.84, Ap3 = 2.00−4.30) but
lower and variable 87Sr/86Sr ratios in hydrothermal
Ap2 (0.7100−0.7165) and Ap3 (0.7086−0.7116) are mainly ascribed to
extensive fluid-wall rock exchange of Sr during mineralization. It is
therefore suggested that fluid-rock reaction played a vital role in the
observed positive Eu anomalies of hydrothermal apatite. Our study
highlights the usefulness of apatite as a novel and robust
geochronological, geochemical, and isotopic indicator of complex
mineralization processes in hydrothermal gold deposits.
Southward continuation and slab rollback of the Neotethyan
arc−back-arc system: Insights from Eocene mafic intrusions from
North Sumatra, SE Asia
Xiaoqing Yu; Yuzhi Zhang; Xin Qian; Azman A. Ghani; Thomas C. Sheldrick ...
The Neotethyan subduction and collision with the Asian, Indian, and
Australian plates is evidenced by a giant Andean-type arc system and a
remarkable Neotethyan igneous belt along the southwestern Eurasian margin.
However, the southward extent, and cause, of Neotethyan subduction is
poorly understood. Here, a zircon U-Pb dating and whole-rock geochemical
study of two groups of Eocene (50−48 Ma) mafic intrusions from Tangse,
North Sumatra, helps to unravel the Neotethyan subduction story. Group 1
consists of diabase samples, which have mid-oceanic-ridge− and arc-like
geochemical affinities, with low chondrite-normalized La/Yb(cn)
(∼0.6) ratios and a high measured εNd(t) value of +8.7.
Such geochemical signatures indicate that the magmatic rocks were derived
from a refractory depleted mantle source that was metasomatized by slab
melts. Group 2 consists of gabbro-diorites, which are enriched in the
large-ion lithophile elements and depleted in some high field strength
elements. These samples have high La/Yb(cn) (2.9−6.6), low Nb/La (cn) (0.2−0.3), εNd(t) values of +1.1 to
+5.0, and εHf(t) values of +8.8 to +15.3. Group 2
samples were produced by the melting of a depleted mantle wedge
metasomatized by slab fluids. The magmatism provides evidence for the
southern continuation of the Neotethyan igneous belt and tectonic system,
whereby slab rollback triggered melting and upwelling in an arc−back-arc
geodynamic system from South Tibet to SE Asia.
Ultrahigh-pressure mantle metasomatism in continental collision
zones recorded by post-collisional mafic rocks
Mingdi Gao; Haijin Xu; Stephen F. Foley; Junfeng Zhang; Yu Wang
Post-collisional mafic magmas are melts of mixed sources formed by the
interaction between crust and mantle. We report the first identification of
metasomatic sources formed at >200 km in continental collision zones
that led to the Triassic post-collisional Shidao mafic magmas in the Sulu
ultrahigh-pressure terrane, eastern China. The Shidao magmas are
ultrapotassic with low Rb/K2O and Rb/Sr ratios, highlighting a
major contribution of K-richterite to the melts, and with high Dy/Yb ratios
indicating residual garnet in the source. Experiments have demonstrated
that K-richterite coexists with garnet only at pressures higher than 6−7
GPa, which indicates that the Yangtze continental crust was subducted to
>200 km, where it interacted with the overlying mantle to produce
K-richterite + garnet-bearing assemblages. This ultra-deep source of the
Shidao magmas spotlights an alternative mechanism for the genesis of
post-collisional magmas, which are generally of shallow origin.
Petrogenesis and tectonic setting of Neoarchean
tonalitic-trondhjemitic-granodioritic gneisses in the Xiwulanbulang
area of the Yinshan block, North China craton
Xiao Wang; Jian Zhang; Qian Liu; Hai Zhou; Changqing Yin ...
As one of the major Archean microcontinental blocks in the North China
craton, the Yinshan block consists of the Guyang granite-greenstone belt
and the Wuchuan high-grade complex, both of which preserve large volumes of
Neoarchean tonalitic-trondhjemitic-granodioritic (TTG) gneisses. The
petrogenesis and tectonic setting of these TTG gneisses are of great
significance to understanding the Neoarchean crustal growth and evolution
of the Yinshan block. Although extensive investigations have been carried
out on the TTG gneisses in the Yinshan block, controversy remains regarding
their petrogenesis and tectonic setting. In this study, we present new
geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data for the
Neoarchean TTG gneisses in the Xiwulanbulang area from the Wuchuan
high-grade complex. Laser ablation−inductively coupled plasma−mass
spectrometry zircon U-Pb dating results from six TTG gneissic samples show
that their igneous TTG plutons were emplaced in the period 2.61−2.48 Ga.
The results from zircon Lu-Hf isotopes show that εHf(t)
values (−3.2 to +7.5) and depleted mantle model ages (3270−2651 Ma) are
like those of the oldest trondhjemite (ca. 2.7 Ga) exposed in the
Xiwulanbulang area, indicating that it mainly resulted from partial melting
of 2.7 Ga juvenile mafic crust. Geochemically, the Xiwulanbulang TTG
gneisses contain high Sr (280−800 ppm) contents, low Y (1.05−9.54 ppm) and
Yb (0.05−0.90 ppm) contents, and relatively high LaN/Yb N (12.8−105) and Sr/Y (39.91−1392.6 ppm) ratios, with relative
enrichments in light rare earth elements (LREEs) and large ion lithophile
elements (LILEs), showing the characteristics of adakitic rocks. In
addition, their lower contents of MgO, Ni, Cr, and Mg# indicate that the
Xiwulanbulang TTG rocks belong to adakite sourced from thickened
continental lower crust. Also, the TTG gneisses show similar initial87Sr/86Sr ratios (0.701659−0.702635) and εNd(t) values (−4.37 to + 3.55), with 206Pb/ 204Pb(t) =
13.910485−15.241164, 207Pb/204Pb( t) = 14.806723−15.259938, and 208
Pb/204Pb(t) =
33.758621−35.592727, suggesting a depleted mantle source modified with some
enriched components. Collectively, the Xiwulanbulang TTG rocks were most
likely derived from the partial melting of thickened mafic crust between
2.61 and 2.48 Ga. Combining previous studies with data presented in this
paper, we propose that the Yinshan block underwent two major igneous events
during Neoarchean time, with the earlier one occurring ca. 2.7 Ga, forming
a thickened mafic crust and minor TTG rocks, and the second igneous event
at 2.61−2.48 Ga, resulting in the partial melting of the ca. 2.7 Ga
continental lower (mafic) crust to form the TTG plutons. The geochemical
data for the Xiwulanbulang TTG gneisses are inconsistent with a model for
the partial melting of subducted slabs, but combined with other geologic
considerations, a model in which the partial melting of a thickened
continental lower (mafic) crust was heated by a mantle plume is favored for
the formation of these Neoarchean TTG rocks in the Yinshan block.
Episodic magmatism in the Lianhuashan tectonic belt: Implications
for late Mesozoic crustal reworking in SE South China
Yujia Xin; Jianhua Li; Yueqiao Zhang; Shuwen Dong
The Lianhuashan tectonic belt preserves abundant evidence of episodic
magmatism and deformation that is crucial for understanding the late
Mesozoic crustal reworking in SE South China. Here, we report zircon U-Pb
ages, geochemical, and Sr−Nd−Hf isotopic data for representative granitoids
and volcanic rocks to clarify the episodic magmatism in the Lianhuashan
tectonic belt and its association with regional tectonic evolution. Our
results define three episodes of magmatism. The Late Jurassic (160−150 Ma)
magmatic rocks show high CaO/Na2O and molar CaO/(MgO + FeOt)
(CMF) ratios and enriched Sr−Nd−Hf isotopes that are consistent with
high-temperature melting of psammitic sources. The Early Cretaceous
(144−131 Ma) magmatic rocks display variable CaO/Na2O ratios and
zircon εHf(t) values (−12.0 to −2.5), which implies binary
mixing of melts from pelitic and basaltic sources. The Late Cretaceous (ca.
94 Ma) magmatic rocks show high A/CNK and low CaO/Na2O and CMF
ratios, which are characteristics of pelite-derived melts. They have zircon
Hf isotopes (−4.4 to −1.9) that are slightly higher than those of the Early
Cretaceous rocks, which is consistent with partial melting of diverse
sources with more involvement of juvenile crust. These magmatic records,
coupled with regional structural observations, suggest that the Lianhuashan
tectonic belt underwent alternating crustal shortening and extension during
the Late Jurassic to Cretaceous time. The Late Jurassic magmatism possibly
marks the initiation of crustal extension following Middle Jurassic
retro-arc crustal shortening related to the advancing Paleo-Pacific
subduction. In the Cretaceous, the two magmatic flare-ups were separated by
a magmatic lull associated with crustal shortening. The magmatic flare-ups
occurred coevally with two stages of regional extension, as evidenced by
widespread generations of basins and extensional domes. The time
consistency implies that the magmatism was associated with or facilitated
crustal extension that occurred at a regional scale and may have resulted
from slab rollback during subduction retreat. We suggest that the magmatism
and extension provide essential evidence of the crustal reworking in SE
South China governed by the Paleo-Pacific subduction. Changes in the slab
dynamics (steepening and shallowing) may control the flare-up, lull, and
resumption of magmatism, as well as alternating episodes of crustal
extension and shortening.
Orbital and sub-orbital pacing of mudstones in the Dongying
Depression, eastern China: Implications for middle Eocene East
Asian climate evolution
Xiangxin Kong; Zaixing Jiang; Yuan Cai
The middle Eocene was a key phase in East Asian and global climate
evolution; however, the understanding of the astronomical driving
mechanisms of key climate changes during this phase in eastern China is
lacking. A 390-m-thick, continuous lacustrine mudstone succession in the
Dongying Depression (eastern China) provides an ideal middle Eocene
sedimentary record. This study established an astronomical time scale (ca.
38.97−43.55 Ma) for the Dongying mudstones by double tuning of gamma-ray
logs and calcite series. Based on the tuned pollen, geochemical element and
nitrogen isotope data, paleo-humidity and paleo-salinity evolution profiles
were constructed, showing that Dongying mudstones experienced a phased
decrease in salinity and an increase in humidity. The rapid wetting
accompanied by abnormal high salinity during ca. 40.0−40.6 Ma indicates
that Dongying mudstones responded to the middle Eocene Climatic Optimum
(MECO) event. Multi-parameter analysis showed that the rhythmic changes of
Dongying mudstones at different scales were driven by multi-scaled
astronomical cycles. This study focused on the intensity changes of
recorded obliquity and semi-precession signals. The signals of
semi-precession weakened in the dry pre-MECO period, while those of
obliquity weakened in the wetter MECO period. Considering the limitation of
latitude on the influence of the two cycles on the surface sunshine, we
speculate that the signals of obliquity and semi-precession may reflect the
intensities of high-latitude dry airflow and low-latitude wet airflow,
respectively, liking to the current East Asian monsoon (EAM) system. This
study expends understanding of the role of astronomical forcing on the
onset and strengthening of the EAM.
Two stages of porphyry Cu mineralization at Jiru in the Tibetan
collisional orogen: Insights from zircon, apatite, and magmatic
sulfides
Xiang Sun; Jun Deng; Yongjun Lu; Xiaobo Si; Pete Hollings ...
Continental collision−related porphyry copper (Cu) deposits provide
significant global copper resources, but their genesis remains
controversial because it is not clear whether remelting of remnant
arc-derived lower-crustal Cu-rich cumulates is critical to their formation.
We investigated zircon and apatite compositions of the Jiru porphyry Cu
deposit in the Gangdese belt of southern Tibet, which is characterized by
weaker early Eocene mineralization and more pronounced Miocene
mineralization. Our data demonstrate that apatite hosted in zircon can
record the volatile compositions of magma before fluid exsolution.
Zircon-hosted apatites from the early Eocene granite that have low XF/XCl ratios (≤3) and are
interpreted to have crystallized from volatile-undersaturated magma have Cl
contents of 0.96−2.14 wt% (1.86 ± 0.38 wt%; n = 15) and SO 3 contents of 0.01−0.14 wt% (0.08 ± 0.04 wt%; n = 15).
In contrast, zircon-hosted apatites from the Miocene porphyry that have low XF/XCl ratios (≤10) and are
interpreted to have crystallized from volatile-undersaturated magma have Cl
contents of 0.40−0.56 wt% (0.47 ± 0.06 wt%; n = 11) and SO 3 contents of 0.20−0.92 wt% (0.59 ± 0.29 wt%; n = 11).
In addition, the early Eocene magma was relatively reduced (ΔFMQ = 0.86 ±
0.55, where FMQ is fayalite-magnetite-quartz) compared with the oxidized
Miocene magma (ΔFMQ = 2.04 ± 0.43). The contrasting magmatic oxidation
states and apatite/melt S contents were likely critical in controlling the
different scales of Cu mineralization during the early Eocene and Miocene,
as more highly oxidized magma can dissolve much larger amounts of sulfur
and metals (e.g., Cu). Melt Cl content may not play a critical role in
magma fertility, since the Jiru early Eocene granite had higher melt Cl
contents than the Miocene porphyry. Magmatic sulfides (pyrrhotite,
chalcopyrite, and pyrite) were not recognized in the Miocene zircons but
did occur in the early Eocene zircons, and these sulfides were demonstrated
to have crystallized before fluid exsolution in the shallow magma
reservoir. The Jiru early Eocene magmatic sulfide saturation might not have
enhanced the amount of copper available to hydrothermal fluids during the
early Eocene. Late-stage sulfide saturation in the previous arc magmas
might reduce the potential for collision-related porphyry Cu mineralization
that is associated with remelting of the previous arc lower crust.
Fossil divergent double-subduction zone in the Great Xing’an Range,
NE China: Evidence from a deep seismic reflection profile
Xiao-Miao Tan; Rui Gao; Jian-Bo Zhou; Simon A. Wilde; He-Sheng Hou ...
Divergent double-subduction is an important component of plate tectonics
and continental growth at modern active convergent margins. However, such a
process is rarely identified by deep seismic reflection (DSR) profiles,
which casts doubt on its past significance. We report here a 416 km DSR
profile of large dynamite shots across the Great Xing’an Range (NE China),
which shows multiple arc-shaped reflections in the upper-middle crust of
the Xing’an terrane, and densely layered reflection structures suggesting
tectonic nappes in the lower crust. This new evidence, together with recent
geological studies, indicates that the Xing’an terrane may not be a
micro-continental block as previously thought, but a Paleozoic accretionary
complex. In addition, there is a strong west-dipping reflection at the
boundary between the Xing’an and Erguna terranes in the western DSR profile
extending from the lower crust to the lithospheric mantle, suggesting that
this fossil subduction zone is most likely the Toudaoqiao-Xinlin suture,
which separates the Xing’an and Erguna terranes. In addition, an
east-dipping strong reflection was also found in the eastern profile
between the Xing’an and Songliao terranes, corresponding to the
Hegenshan-Nenjiang suture at the surface. Given the newly identified
crust-mantle structure, the opposite subduction zone, and other geological
evidence, we propose a new model that the Xing’an terrane was developed by
divergent double-subduction of the Paleo-Xing’an Ocean (part of the
Paleo-Asian Ocean in the area) from ca. 500 Ma to ca. 300 Ma. Our study
provides the first DSR evidence for a divergent double-subduction in the
Paleozoic, which emphasizes its importance in past accretionary orogens.
Diachronous subduction, closure of the Proto-Tethys Ocean, and
collisional accretion of microcontinents: Insights from the early
Paleozoic intermediate-mafic rocks in the Amdo microcontinent of
the Tibetan Plateau
Yinbiao Peng; Shengyao Yu; Sanzhong Li; Yongjiang Liu; Pei Lv ...
The Proto-Tethys Ocean played an important role in the formation and
evolution of the East Asian continental blocks, and the final closure of
the Proto-Tethys Ocean may have led to the first assembly of nearly all
East Asian blocks at the northern margin of Gondwana. However, controversy
remains about the reconstruction of East Asian blocks at the northern
margin of Gondwana. The Tibetan Plateau is located in the northern margin
of Gondwana and recorded the entire tectonic evolution from continental
rift and drift to subduction and collision during the early Paleozoic.
Therefore, it is an important region in which to address tectonic evolution
processes of the Proto-Tethys Ocean and Gondwana. Here, we present
petrology, zircon U-Pb, Sr-Nd-Hf isotope, and whole-rock geochemical data
from newly recognized intermediate-mafic magmatic rocks from the Amdo
microcontinent of central Tibet, with a view to gain insights into the
nature and geotectonic evolution of the northern margin of Gondwana. Zircon
grains from meta-andesites and meta-basites in Amdo yielded magmatic
crystallization ages of 490 Ma and 455−450 Ma, respectively. The
meta-andesites are characterized by enrichment in large ion lithophile
elements (e.g., Rb, Th, U, and K) and depletion in high field strength
elements (e.g., Nb, Ta, and Ti) with positive zircon εHf( t) values (+3.4 to +6.4), which were probably derived from partial
melting of a depleted mantle source enriched by minor fluids from subducted
sediments in a continental arc setting. The meta-basites are tholeiitic and
exhibit both mid-ocean-ridge basalt (e.g., flat rare earth element
patterns) and arc-like (e.g., elevated Th/Yb ratios) geochemical
affinities, in combination with negative zircon εHf(t)
(−4.6 to −0.3) and whole-rock εNd(t) (−0.21 to −0.45)
values, suggesting that they were probably generated by varying degrees of
partial melting of a spinel peridotite source with ∼5%−10% crustal
assimilation in a back-arc setting. In this regard, we favor the
interpretation that the Amdo microcontinent experienced long-lasting (ca.
530−450 Ma) subduction of the Proto-Tethys Ocean slab before the subsequent
collisional accretion of microcontinents. Integrating results from previous
studies, we propose that the Amdo microcontinent, South Qiangtang terrane,
Qilian-Qaidam terrane, and other East Asian blocks were all located at the
northern margin of East Gondwana, which recorded the arc-related magmatism
along the proto-Tethyan margin in the early Paleozoic. The closure of the
Proto-Tethys Ocean and initial collision along the East Gondwana
Proto-Tethyan margin may have been diachronous between ca. 455 and 430 Ma.
Large detrital zircon data set investigation and provenance
mapping: Local versus regional and continental sediment sources
before, during, and after Ancestral Rocky Mountain deformation
Tyson M. Smith; Joel E. Saylor; Tom J. Lapen; Ryan J. Leary; Kurt E.
Sundell
The increasing number and size of detrital geochronology data sets offer
new opportunities for increased accuracy and resolution of sediment routing
models. However, the new opportunities come coupled with challenges in
large data integration and visualization. We address these challenges by
outlining two novel approaches that aid in analyzing and interpreting large
detrital geochronology data sets: (1) combination of bottom-up and top-down
detrital zircon source modeling, and (2) sediment provenance mapping.
Combining source-modeling methods provides guidance in identifying
empirical detrital zircon sources and determining source proportions.
Provenance mapping integrates source proportions from modeling results and
complimentary geologic data (e.g., paleocurrents, paleogeography, and
stratal thickness maps) to extrapolate provenance information through areas
with sparse or ambiguous data, thus mitigating issues of data distribution
heterogeneity. Sediment provenance maps also provide a synoptic view of
data that, along with detrital zircon source modeling, aids in
circumventing lengthy descriptions of individual age modes for data sets
containing hundreds of samples, which can obscure underlying trends in the
data. We apply this approach to late Paleozoic−early Mesozoic strata, using
329 published and new U-Pb detrital zircon samples, and document five
sediment-routing episodes in the core zone of intraplate deformation in
western Laurentia (i.e., the Ancestral Rocky Mountains (ARM)). The
transitions between these episodes are defined by changes in sediment
source distribution, which are illustrated by provenance maps that show (1)
the degree and extent of ARM basin isolation from transcontinental sediment
sources and (2) ARM-driven changes in transcontinental sediment routing
systems. We map possible sediment pathways of distally derived sediment
around the ARM core, illustrating that ARM uplifts diverted
transcontinental systems around areas of intense intraplate deformation.
Further, the evolution of sediment routing in western Laurentia before,
during, and after ARM deformation provides an example of the interaction
between transcontinental sediment routing and intraplate deformation.
Bifurcation of mantle plumes by interaction with stagnant slabs in
the mantle transition zone: Evidence from late Cenozoic basalts
within Southeast Asia
Fan Yang; Xiao-Long Huang; Yi-Gang Xu; Peng-Li He
Mantle plumes can encounter stagnant slabs in the mantle transition zone
(MTZ) during their ascent from the lower mantle to Earth’s surface.
However, the interactions between stagnant slabs and mantle plumes remain
poorly understood. This study presents new data on the spatial
thermal−lithological−chemical heterogeneity in the mantle over a vast area
of Southeast Asia that reflects the variable influence of a late Cenozoic
mantle plume. Site U1433 of International Ocean Discovery Program (IODP)
Expedition 349 in the southwest sub-basin of the South China Sea (SCS) is
located near the center of this seismically imaged mantle upwelling. This
area also contains mid-ocean ridge basalt (MORB) derived from a
peridotite-dominated and normal temperature mantle source with a depleted
Nd-Hf and enriched 207Pb/206Pb isotopic composition.
In contrast, MORB from Site U1431 of IODP Expedition 349 within the eastern
sub-basin of the SCS and other intraplate basalts from the Hainan-Leizhou
area, the eastern Indochina block, and elsewhere in the SCS are dominantly
sourced from higher temperature pyroxenite-enriched mantle material that
records mantle plume activity. These observations are in contrast with
previous models involving a single plume rising from the lower mantle
beneath the SCS that then tilted toward Hainan Island, China. Instead, it
is likely that the observations from this region are indicative of
plume-stagnant slab interaction. The shallow upper mantle beneath Site
U1433 has not been influenced by a mantle plume as the presence of stagnant
slabs within the MTZ prevented further upwelling of the plume. In contrast,
a mantle plume passed through the weaker MTZ within the eastern sub-basin
and circum-SCS areas, allowing plume-related magmatism to occur. The
bifurcation of a deep-rooted mantle plume by stagnant slabs in the MTZ
produced a cluster of small mantle upwellings that ascended to shallow
depths, consistent with new multiscale global tomographic data for
Southeast Asia.
Genesis and source affinities of heterogeneous ultramafic rocks in
the North Kongling Complex, Yangtze craton: Architecture of a
Paleoproterozoic accretionary orogen
Hongtao Peng; Hao Deng; Qingsen Han; Songbai Peng; Ali Polat ...
Ultramafic massifs are minor but meaningful components of orogens because
they record the mantle properties and the processes of plate generation,
subduction, accretion, and collision, thus providing key information about
crust-mantle interaction processes and the evolution of orogens. Here, we
present an integrated study of field observations, whole-rock and mineral
geochemistry, zircon U-Pb ages, and whole-rock Sm-Nd and Re-Os isotopes for
serpentinized harzburgites and olivine pyroxenites from the
Paleoproterozoic Shuiyuesi mélange belt, North Kongling Complex, to
constrain the tectonic evolution of the Yangtze craton, South China. The
serpentinized harzburgites are characterized by enrichment of light rare
earth elements (LREEs) and negative εNd(t) (−4.3 to
−3.9) values, but they have high olivine Mg# (91.7−93.1) and very low Re
contents (<0.09 ppb) and negative γOs(t) (−5.1 to
−4.0) values. The Archean Re-Os model ages (TRD = 2.82 Ga) of
the harzburgites that have the most depleted Re-Os isotope composition
represent estimates for the age of the initial melt extraction. Metasomatic
zircons from the serpentinized harzburgites yield a weighted average age of
ca. 2.04 Ga, which is interpreted to represent the age of metasomatism
during a subduction event. These geochemical and isotopic features and
geochronologic results suggest that the protolith of the serpentinized
harzburgites in the North Kongling Complex originated as fragments of late
Mesoarchean (ca. 2.82 Ga) subcontinental lithospheric mantle and
experienced melt/fluid metasomatism in a subarc mantle wedge at 2.04 Ga.
The olivine pyroxenites are cumulates, and they are characterized by high
MgO, Ni, and Cr contents and depleted to slightly enriched LREEs but
depletion in high field strength elements (HFSEs). They have positive ε Nd(t) (+2.0 to +6.6) values and Paleoproterozoic Nd
model ages (2.41−2.05 Ga), suggesting that the olivine pyroxenites were
derived from a Paleoproterozoic depleted mantle source. Metamorphic zircons
from the olivine pyroxenites indicate subsequent metamorphism at 2.0 Ga, in
response to a collision. The olivine pyroxenites together with the
metabasites in the Shuiyuesi mélange belt represent the dismembered
fragments of oceanic lithosphere that formed in a suprasubduction zone
during the Paleoproterozoic. Combined with the Archean and Paleoproterozoic
rocks in the North Kongling Complex, it appears that the Mesoarchean
subcontinental lithospheric mantle peridotites and Paleoproterozoic
ophiolitic rocks were simultaneously accreted during a Paleoproterozoic
orogenic event. We further propose that a subduction channel may have
played an important role in the architecture of the accretionary orogen, in
which both oceanic lithospheric materials and subcontinental lithospheric
mantle wedge materials would have been simultaneously scraped off and
accreted during slab subduction-accretion.
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