New GSA Bulletin Articles Published Ahead of Print in October
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; the Dead Sea
transform; and volcanism in an extensional basin along the Laurentian
Iapetus margin. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent.
Short-lived intra-oceanic arc-trench system in the North Qaidam belt
(NW China) reveals complex evolution of the Proto-Tethyan Ocean
Changlei Fu; Zhen Yan; Jonathan C. Aitchison; Wenjiao Xiao; Solomon Buckman
...
Abstract:
Recognition of any intra-oceanic arc-trench system (IOAS) could provide
invaluable information on the tectonic framework and geodynamic evolution
of the vanished ocean basin. The Tanjianshan Complex and mafic-ultramafic
rocks along the North Qaidam ultra-high pressure metamorphic belt in NW
China record the subduction process of the Proto-Tethyan Ocean. Four
lithotectonic units, including island arc, ophiolite, forearc basin, and
accretionary complex, are recognized based on detailed field investigation.
They rest on the northern margin of the Qaidam block and occur as
allochthons in fault contact with underlying high-grade metamorphic rocks.
The ophiolite unit mainly consists of ultramafic rocks, 527−506 Ma gabbro,
515−506 Ma plagiogranite, dolerite, and massive lava. High-Cr spinels in
serpentinite, dolerite with forearc basalt affinity, and boninitic lava
collectively indicate a forearc setting. The accretionary complex, exposed
to the south of the ophiolite complex and island arc, is highly disrupted
and contains repeated slices of basalt, 495−486 Ma tuff, chert, limestone,
and mélange. Tuffs with positive zircon εHf(t) values indicate
derivation from a nearby juvenile island arc. These lithotectonic units, as
well as the back-arc basin, are interpreted to constitute a Cambrian IOAS
that formed during the northward subduction of the Proto-Tethyan Ocean.
Combined with regional geology, we propose a new geodynamic model involving
short-lived Mariana-type subduction and prolonged Andean-type subduction to
account for the complex evolution of the Proto-Tethyan Ocean. The
reconstruction of a relatively complete IOAS from the North Qaidam belt not
only reveals a systematic evolution of intra-oceanic subduction but also
advances our understanding of the subduction and accretion history of the
Proto-Tethyan Ocean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36127.1/609274/Short-lived-intra-oceanic-arc-trench-system-in-the
Multiple sources and magmatic evolution of the Late Triassic Daocheng
batholith in the Yidun Terrane: Implications for evolution of the
Paleo-Tethys Ocean in the eastern Tibetan Plateau
Pengsheng Dong; Guochen Dong; M. Santosh; Xuanxue Mo; Peng Wang ...
Abstract:
Granitoids with diverse composition and tectonic settings provide important
tools for exploring crustal evolution and regional geodynamic history. Here
we present an integrated study using petrological, mineralogical, zircon
U-Pb geochronological, whole-rock geochemical, and isotopic data on the
Late Triassic Daocheng batholith in the Yidun Terrane with a view to
understanding the petrogenesis of a compositionally diverse batholith and
its implications for the evolution of the Paleo-Tethys Ocean in the eastern
Tibetan Plateau. The different lithological units of the batholith,
including granodiorite, monzogranite, and quartz diorite, with abundant
mafic microgranular enclaves in the granodiorite (MME I) and monzogranite
(MME II), show identical crystallization ages of 218−215 Ma. The mineral
assemblage and chemical composition of the granodiorite are identical to
those of tonalitic-granodioritic melts generated under water-unsaturated
conditions. The insignificant Eu anomalies and low magmatic temperatures
indicate hydrous melting in the source. The relatively narrow range of
whole-rock chemical and Sr-Nd isotopes, as well as the zircon trace element
and Hf isotopic compositions of the granodiorite, suggest a homogeneous
crustal source for the magma. Our modeling suggests that the rock was
produced by 20−50% of lower crustal melting. The Daocheng monzogranites
display more evolved compositions and larger variations in Sr-Nd-Hf
isotopes than the granodiorite, which are attributed to assimilation and
the fractional crystallization process. This is evidenced by the presence
of metasedimentary enclave and inherited zircon grains with Neoproterozoic
and Paleozoic ages, a non-cotectic trend in composition, and the trend
shown by the modeling of initial 87Sr/86Sr ratios and
Sr. The quartz diorites and MMEs showing composition similar to that of
andesitic primary magma have high zircon εHf(t) values and are
characterized by enrichment in LILEs and depletion of HFSEs. They were
derived from the partial melting of lithospheric mantle that had been
metasomatized by slab melts and fluids. The MMEs in both rocks display
typical igneous texture and higher rare earth element (REE) and
incompatible element concentrations than their host granites. The presence
of fine-grained margins, acicular apatite, and plagioclase megacrysts
suggests a magma mingling process. The overgrowth of amphibole around the
pyroxene, quartz ocelli rimmed by biotite, and oscillatory zones of
plagioclase are all indicative of chemical diffusion. Their enriched Sr-Nd
isotopes imply isotopic equilibrium with the host granites. Based on a
comparison with the coeval subduction-related magmatism, we propose that
subduction and subsequent rollback of the Paleo-Tethys (Garzê-Litang Ocean)
oceanic slab was the possible mechanism that triggered the diverse Triassic
magmatism within the eastern Tibetan Plateau.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35970.1/609047/Multiple-sources-and-magmatic-evolution-of-the
Basin response to the Jurassic geodynamic turnover from flat subduction
to normal subduction in South China
Wei Zhang; Feng-Qi Zhang; Yildirim Dilek; Kong-Yang Zhu; Hong-Xiang Wu ...
Abstract:
Growing evidence supports that the early Mesozoic development of South
China was deeply shaped by flat subduction of the paleo-Pacific plate.
However, the Jurassic tectonic setting and processes remain controversial.
Here, we present new detrital zircon U-Pb ages and sedimentary data from
the Jurassic basin in northwestern Zhejiang to constrain the Jurassic
tectonic evolution. The continuous Jurassic succession archives an abrupt
sedimentary change from a high-energy coastal environment to a proximal and
fast-filling terrestrial environment. This lithostratigraphic change was
also accompanied by the shift of detrital provenance. Age spectra of the
detrital zircons from the bottom of the Jurassic strata show strong 1.0−0.7
Ga and 500−400 Ma populations, which are inferred to be mainly derived from
the Yangtze block. In contrast, samples from the overlying Lower−Middle
Jurassic were dominated by age groups of 2.0−1.7 Ga and 300−170 Ma, which
were probably sourced from the Cathaysia block. The switch of the
sedimentary and provenance characteristics reveals that an earliest
Jurassic broad sag basin in the inland shifted to an Early−Middle Jurassic
retro-arc foreland basin along the coastal region. Abundant Jurassic-aged
zircons are compatible with the re-initiation of “normal subduction” in the
Early Jurassic. The re-initiation of “normal subduction” resulted in the
generation of an accretionary orogeny, continental arcs, and a retroarc
foreland basin along the eastern South China margin in contrast to the
extensional regime in the inland. The basin response and distinct tectonic
regimes of the inland and continental margin in the Early−Middle Jurassic
support a geodynamic turnover from flat to normal subduction.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36059.1/608457/Basin-response-to-the-Jurassic-geodynamic-turnover
The Mesozoic magmatic, metamorphic, and tectonic evolution of the
eastern Gangdese magmatic arc, southern Tibet
Ze-Ming Zhang; Hui-Xia Ding; Xin Dong; Zuo-Lin Tian; Richard M. Palin ...
Abstract:
Magmatic arcs are natural laboratories for studying the growth of
continental crusts. The Gangdese arc, southern Tibet, is an archetypal
continental magmatic arc that formed due to Mesozoic subduction of the
Neo-Tethyan oceanic lithosphere; however, its formation and evolution
remain controversial. In this contribution, we combine newly reported and
previously published geochemical and geochronological data for Mesozoic
magmatic rocks in the eastern Gangdese arc to reveal its magmatic and
metamorphic histories and review its growth, thickening, and fractionation
and mineralization processes. Our results show that: (1) the Gangdese arc
consists of multiple Mesozoic arc-type magmatic rocks and records
voluminous juvenile crustal growth. (2) The Mesozoic magmatic rocks
experienced Late Cretaceous granulite-facies metamorphism and partial
melting, thus producing hydrous and metallogenic element-rich migmatites
that form a major component of the lower arc crust and are a potential
source for the Miocene ore-hosting porphyries. (3) The Gangdese arc
witnessed crustal thickening and reworking during the Middle to Late
Jurassic and Late Cretaceous. (4) Crystallization-fractionation of
mantle-derived magmas and partial melting of thickened juvenile lower crust
induced intracrustal chemical differentiation during subduction. We suggest
that the Gangdese arc underwent the following main tectonic, magmatic, and
metamorphic evolution processes: normal subduction and associated
mantle-derived magmatism during the Late Triassic to Jurassic; shallow
subduction during the Early Cretaceous and an associated magmatic lull; and
mid-oceanic ridge subduction, high-temperature metamorphism and an
associated magmatic flare-up during the early Late Cretaceous, and flat
subduction, high-temperature and high-pressure metamorphism, partial
melting, and associated crust-derived magmatism during the late Late
Cretaceous. Key issues for further research include the temporal and
spatial distributions of Mesozoic magmatic rocks, the evolution of the
components and compositions of arc crust over time, and the metallogenic
processes that occur in such environments during subduction.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36134.1/608458/The-Mesozoic-magmatic-metamorphic-and-tectonic
Evaluation of the exploration prospect and risk of marine gas shale,
southern China: A case study of Wufeng-Longmaxi shales in the Jiaoshiba
area and Niutitang shales in the Cen’gong area
Qiyang Gou; Shang Xu; Fang Hao; Yangbo Lu; Zhiguo Shu ...
Abstract:
The Wufeng-Longmaxi shales and the Niutitang shales are the most important
organic-rich marine shales in southern China. To fully understand the
significant difference in drilling results between the two sets of shales,
the accumulation conditions of shale gas were systematically compared. The
Niutitang shales have a superior material base of hydrocarbon generation
for higher total organic carbon than the Wufeng-Longmaxi shales. Due to the
influence of hydrothermal activities and carbonization of organic matter,
however, the porosity, pore volume, pore size, and pore connectivity of
Niutitang shales is obviously lower than that of Wufeng-Longmaxi shales.
The natural fractures of Wufeng-Longmaxi shales are dominated by horizontal
bedding fractures, and most of them are filled by calcite. By contrast, the
high dip-angle fractures are more developed in the Niutitang shales.
Especially, these fractures remain open in stages during the process of
serious uplift and denudation movements. Thus, the seal conditions of the
Niutitang shales are poor, which is further not conducive to the enrichment
of shale gas. Our work also suggests that the exploration and development
of highly over matured marine shales in southern China should follow the
principle of “high to find low, and strong to find weak.”
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36063.1/608459/Evaluation-of-the-exploration-prospect-and-risk-of
Identification of ca. 520 Ma mid-ocean-ridge−type ophiolite suite in
the inner Cathaysia block, South China: Evidence from shearing-type
oceanic plagiogranite
Longming Li; Shoufa Lin; Guangfu Xing; Fan Xiao; Wenjiao Xiao
Abstract:
An ophiolite suite, predominantly composed of residual mantle peridotites,
mid-ocean-ridge basalt (MORB)−like ultramafic rocks, and oceanic
plagiogranites, has been identified in the Zhenghe-Dapu fault zone,
Cathaysia block, South China. The peridotites experienced strong
serpentinization and are characterized by low 187Os/188Os ratios of 0.11621−0.12008 and very low 187Re/ 188Os values of 0.031−0.129, similar to those from highly
refractory mantle residues. The meta-ultramafic rocks, mainly amphibolites,
can be classified into two groups. Group I is characterized by high Ni and
Cr and low K2O contents. Their spoon-like rare earth element
(REE) patterns, along with lower concentrations of highly incompatible
elements, indicate that the protolith was of cumulate origin. Group II
displays depleted REE patterns and low Nb/Yb, Th/Yb, and Ti/V ratios, which
are geochemically similar to normal (N) MORB. Both groups exhibit positive
εNd(t) values (1.2−4.1) and relatively high ( 87Sr/86Sr)i ratios (0.7046−0.7096),
suggesting their origin from partial melting of depleted mantle sources in
a mid-ocean-ridge setting that experienced a greater extent of fluid-rock
alteration. The meta-plagiogranites intercalated with the mylonitic
amphibolites are characterized by low K2O (0.09−0.21 wt%) and
total REE contents along with low K2O/Na2O and Rb/Sr
ratios, which are consistent with those of typical oceanic plagiogranite.
They exhibit strongly positive zircon εHf(t) values
(+9.5 to +15.1) and positive whole-rock εNd(t) values
(+2.8 to +3.6). Their extremely low MgO (0.6−1.65 wt%), Cr (0.22−6.26 ppm),
and Ni (0.77−4.74 ppm) compositions and low Mg# (22.4−31.9) preclude their
origination from mantle-derived primary magma but favor oceanic crust. Low
zircon δ18O values (4.02‰−5.4‰) and decoupled Sr-Nd isotope
features imply the involvement of high-temperature seawater alteration in
their source region. The enriched light rare earth element (LREE) patterns
with strongly positive Eu anomalies, similar to the East Karmøy−type
plagiogranite in western Norway, imply that the plagiogranites were derived
from anatexis of amphibolite in an active shear zone near the mid-ocean
ridge. The magmatic and metamorphic zircons from the meta-plagiogranites
yield nearly identical secondary ion mass spectrometry (SIMS) U-Pb ages
ranging from 523 to 521 Ma and from 522 to 518 Ma, respectively. The
simultaneous magmatism and metamorphism also signify an active
high-temperature shear zone, where the plagiogranites were formed slightly
later than the oceanic crust. The age of ca. 520 Ma represents the
formation age of the oceanic crust rather than its emplacement age. The
identification of the ca. 520 Ma ophiolite suite along the Zhenghe-Dapu
fault zone indicates that the Cathaysia block consisted of at least two
different terranes rather than a single tectonic unit in the Cambrian, and
the final amalgamation of the eastern and western Cathaysia block may have
occurred far later than ca. 520 Ma, most likely during the late early
Paleozoic.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36088.1/608460/Identification-of-ca-520-Ma-mid-ocean-ridge-type
U-Pb speleothem geochronology reveals a major 6 Ma uplift phase along
the western margin of Dead Sea Transform
O. Chaldekas; A. Vaks; I. Haviv; A. Gerdes; R. Albert
Abstract:
The timing of vertical motions adjacent to the Dead Sea Transform plate
boundary is not yet firmly established. We utilize laser
ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb
geochronology of carbonate cave deposits (speleothems) to constrain
paleo-groundwater levels along the western margin of the Dead Sea Transform
and provide a proxy for the timing of large-scale incision and tectonic
uplift. Phreatic speleothems can form in caves that are located slightly
below the groundwater level. Tectonic uplift and/or base level subsidence
can trigger incision of canyons and induce a drop in the groundwater table.
This can cause dewatering of the caves, cessation of the deposition of
phreatic speleothems, and initiation of growth of vadose speleothems. The
transition between deposition of phreatic and vadose speleothems can
therefore reflect tectonic or erosive events. We obtained 102 U-Pb ages
from 32 speleothems collected from three cave complexes across a
150-km-long, north-to-south transect. These ages indicate that phreatic
deposition began between 14.68 ± 1.33 and 11.34 ± 1.62and ended by 6.21 ±
0.59 Ma. Later, vadose speleothems grew intermittently until the
Quaternary. These results suggest an abrupt drop in the water table
starting at ca. 6 Ma with no re-submergence of the caves. We interpret this
to indicate river incision of ∼150−200 m that was driven by uplift and
folding of the western margin of the Dead Sea Transform and by inland
morpho-tectonic, base-level subsidence in the Dead Sea area. The observed
timing corresponds with a change in the Euler pole of the plates motion
along the Dead Sea Transform. The growth period of phreatic speleothems
suggests groundwater level stability and limited vertical tectonic motions
between 14 Ma and 6 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36051.1/608120/U-Pb-speleothem-geochronology-reveals-a-major-6-Ma
Concurrent MORB-type and ultrapotassic volcanism in an extensional
basin along the Laurentian Iapetus margin: Tectonomagmatic response to
Ordovician arc-continent collision and subduction polarity flip
Deta Gasser; Tor Grenne; Fernando Corfu; Reidulv Bøe; Torkil S. Røhr ...
Abstract:
Arc-continent collision, followed by subduction polarity flip, occurs
during closure of oceanic basins and contributes to the growth of
continental crust. Such a setting may lead to a highly unusual association
of ultrapotassic and mid-ocean ridge basalt (MORB)-type volcanic rocks as
documented here from an Ordovician succession of the Scandinavian
Caledonides. Interbedded with deep-marine turbidites, pillow basalts evolve
from depleted-MORB (εNdt 9.4) to enriched-MORB (εNdt
4.8) stratigraphically upward, reflecting increasingly deeper melting of
asthenospheric mantle. Intercalated intermediate to felsic lava and
pyroclastic units, dated at ca. 474−469 Ma, are extremely enriched in
incompatible trace elements (e.g., Th) and have low εNdt (−8.0
to −6.6) and high Sri (0.7089−0.7175). These are interpreted as
ultrapotassic magmas derived from lithospheric mantle domains metasomatized
by late Paleoproterozoic to Neoproterozoic crust-derived material (isotopic
model ages 1.7−1.3 Ga). Detrital zircon spectra reveal a composite source
for the interbedded turbidites, including Archean, Paleo-, to
Neoproterozoic, and Cambro-Ordovician elements; clasts of Hølonda
Porphyrite provide a link to the Hølonda terrane of Laurentian affinity.
The entire volcano-sedimentary succession is interpreted to have formed in
a rift basin that opened along the Laurentian margin as a result of slab
rollback subsequent to arc-continent collision, ophiolite obduction and
subduction polarity flip. The association of MORBs and ultrapotassic rocks
is apparently a unique feature along the Caledonian-Appalachian orogen.
Near-analogous modern settings include northern Taiwan and the Tyrrhenian
region of the Mediterranean, but other examples of strictly concurrent MORB
and ultrapotassic volcanism remain to be documented.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36113.1/608121/Concurrent-MORB-type-and-ultrapotassic-volcanism
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