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; the Lower
Mississippi Valley, USA; and the polarity of Mesozoic arcs along the
western margin of North America. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent.
Magmatic-hydrothermal evolution of long-lived Nb-Ta-(Sn)
mineralization in Lianyunshan, NE Hunan, South China
Nuerkanati Madayipu; Huan Li; Safiyanu Muhammad Elatikpo; Michael W.
Förster; Hou-Xiang Zhou ...
The Lianyunshan Complex (LYSC) is a Nb-Ta-Li-Be rare metals mining area in
northeastern Hunan, South China. The deposit is composed of a granite-type
rare-metal pegmatite mineralization that occurs as dikes around the LYSC.
The present study examines the genetic relationship, fluid evolution, and
metallogeny of the co-developed ore-free pegmatite (OFP) and ore-bearing
pegmatite (OBP) in granite-related pegmatite-type Nb-Ta rare-metal deposits
in the LYSC. Four minerals: columbite-tantalite (coltan), zircon,
cassiterite, and monazite were investigated for their chemical and isotopic
compositions. The coltan grains range from homogeneous crystals to zoned
varieties with growth rims that result from equilibrium and disequilibrium
reactions due to local changes in the physical and chemical conditions.
These structures were altered by later metasomatic replacement processes
related to the hydrothermal fluid activity and produced structures such as
alteration rims, patches, irregular zonations, and complex zonations, with
a corresponding increase in Ta. Coltan in OBP yielded two weighted mean 206Pb/238U ages of 135.9 ± 2.1 Ma and 120.5 ± 1.2 Ma,
corresponding to magmatic (early-stage) and hydrothermal (late-stage) Nb-Ta
mineralization ages, respectively. The cassiterite gives a weighted mean
U-Pb mineralization age of 130.5 ± 3.9 Ma, which is interpreted as the
magmatic-hydrothermal age (transitional stage). Zircon grains in OBP give a
lower weighted mean U-Pb age of 121.5 ± 0.8 Ma, resulting from
metamictization by the later fluid-assisted alteration and
recrystallization in the late-stage hydrothermal event. Monazite grains
from OFP yielded a weighted mean 206Pb/238U age of
137.1 ± 0.5 Ma, while zircon gives a mean U-Pb age of 130.3 ± 0.6 Ma. The
ages of 137 and 130 Ma obtained for the OFP represent the magmatic
(early-stage) and magmatic-hydrothermal stage (transitional) of OFP
crystallization in the LYSC. Calculated zircon εHf (t) values and T DM2 ages from OFP (−7.6 to −5.4 and 1663−1532 Ma, respectively)
and OBP (−8.7 to −8.2 and 1735−1704 Ma, respectively) are akin to those of
the schist and metasandstone strata of the metasedimentary Lengjiaxi
metasedimentary group—a melt source material to the LYSC composite
batholith. We propose a long-lived ∼15 m.y. Nb-Ta-(Sn) mineralization model
for the Lianyunshan complex.
Provenance and evolution of East Asian large rivers recorded in the
East and South China Seas: A review
Licheng Cao; Lei Shao; Douwe J.J. van Hinsbergen; Tao Jiang; Di Xu ...
Large rivers are the arteries of continents. Those originating from the
Tibetan Plateau and traversing East Asia have a relatively young history
due to continuous Cenozoic perturbations. However, it has been a long
journey to reconstruct their genesis and dynamic evolution, in which many
puzzles and challenges remain. The river history is documented by
provenance information in the ultimate sediment sinks in the East and South
China Seas, but a regional-scale correlation of provenance data is still
developing. Here, we explore the promise of this provenance perspective by
reconstructing the evolution of three large rivers in China (the Yangtze,
Pearl, and Red Rivers) by compiling and reevaluating a large volume of
published provenance data (zircon U-Pb geochronology, K-feldspar Pb
isotopes, and whole-rock Nd isotopes) from both Cenozoic strata and modern
sediments from the East and South China Seas and the large river basins.
Unlike traditional approaches that average provenance signatures,
intersample variability was carefully evaluated. The general inheritance of
zircon age spectral patterns and small fluctuations of Nd isotopes in the
Neogene strata suggest provenance stabilization in the East and South China
Seas and the establishment of near-modern drainage configurations. The
paleodrainage basins before the Miocene are interpreted to have been
smaller than their modern sizes, and drainage expansion likely occurred
over the Oligocene. Our analysis suggests that the widely accepted models
that link drainage between the ancient Yangtze and Red Rivers may be
unlikely. The compiled provenance signatures and prior paleocurrent
measurements of Paleogene strata distributed in the southeastern Tibetan
Plateau margin show sediment supplied from local terranes instead of
through-flowing river systems.
Climate and anthropogenic impacts on North American erosion and
sediment transport since the Last Glacial Maximum: Evidence from
the detrital zircon record of the Lower Mississippi Valley, USA
Nikki Neubeck; Andrew Carter; Tammy Rittenour; Peter D. Clift
The Mississippi River provides an opportunity to examine models of sediment
transport in large alluviated floodplain systems. We test the idea that
sources of sandy sediment in such settings are invariable on timescales
<104 y because of storage and recycling in the floodplains.
To reconstruct the development of the Mississippi sediment load over the
past 2500 years we collected sediment from an abandoned point bar complex
nearby at False River, Louisiana, USA. We also took annual samples from the
lower reaches between 2015 and 2021 to assess changes on that timescale.
Optically stimulated luminescence dating indicated that the point bar
accreted between 2460 and 860 years ago. Detrital zircon U-Pb dating was
used to assess sediment source and variability over time. We confirm a
dominant sediment flux from the Rocky Mountain foreland but with higher
relative erosion from the Superior Province during the Last Glacial Maximum
(LGM) based on existing data from the Gulf of Mexico. There have been
resolvable changes in the sources of sediment particularly since the LGM
and after 860 years ago, but also over shorter, even sub-annual timescales
in the recent past. These changes may reflect seasonal weather or storm
events in the headwater regions and imply limited floodplain buffering of
the sand load. In recent times this may reflect the installation of levees
in the lower reaches, suppressing reworking. Changes over 102−10 3 y time periods may be related to changes in climate (e.g., the
Medieval and Roman warm periods) and to the development of agriculture
across North America after ∼2000 years ago. Detrital zircon dating is an
effective provenance tool and does not appear to be strongly biased by the
grain size of the sediment in this setting.
Accretion of an early Paleozoic Alaska-type arc onto northern North
China: Implications for continental growth of the Central Asian
orogenic belt
Hao Zeng; Dongfang Song; Wenjiao Xiao; Puqing Li
The accretionary orogenesis of the Central Asian orogenic belt is essential
for understanding the reconstruction and growth of the Asian continent. The
origin and accretion of the Bainaimiao arc along the northern margin of
North China remain controversial. Here, a comprehensive study of field
geology, geochemistry, and geochronology was performed on plutons and an
ophiolite-bearing accretionary complex in the Ganqimaodu region, Inner
Mongolia. Geochemical data indicate that ca. 445−431 Ma plutons were
generated in an intra-oceanic arc and represent the western extent of the
Bainaimiao arc. Field mapping and zircon U-Pb dating revealed that the
ophiolitic mélange contains ca. 470−435 Ma tectonic blocks of ultramafic
rocks, gabbros, plagiogranites, and metavolcanic and siliceous rocks in a
matrix of intensely foliated quartz schist. The pillow basalt−limestone
sequence might have originated from a seamount and then been incorporated
into the accretionary complex. Northward subduction of the South Bainaimiao
Ocean was responsible for the early Paleozoic tectonic evolution of the
Bainaimiao arc, based on the spatial-temporal configuration of arc
magmatism and accretionary complexes. Integration of new data with previous
studies indicates that the Bainaimiao arc was an Alaska-type arc with
various components of intra-oceanic and continental arcs. We suggest that a
scissor-like arc-continent collision led to the accretion of the Bainaimiao
arc onto the northern margin of North China, which played a fundamental
role in the continental growth of the southern Central Asian orogenic belt.
Re-appearance of precipitated aragonite crystal fans as evidence
for expansion of oceanic dissolved inorganic carbon reservoir in
the aftermath of the Lomagundi-Jatuli Event
Guang Ouyang; Zhenbing She; Qian Xiao; Kenan Qian; Zongyuan Kenan ...
The initial accumulation of atmospheric oxygen is marked by the
unprecedented positive δ13Ccarb excursions of the
Lomagundi-Jatuli Event (LJE) and records an interval of abnormal O 2 production through elevated rates of organic carbon burial.
Emerging evidence suggests that the post-LJE atmosphere-ocean system might
have suffered a significant deoxygenation. These dynamic perturbations in
the oceanic redox state and biogeochemical cycles would have led to
fundamental changes in carbonate precipitation dynamics. Here, we report
the discovery of centimeter-sized crystal fans in the post-LJE Huaiyincun
Formation, Hutuo Supergroup in the North China Craton. The hexagonal
cross-sections and square terminations suggest that these fan-like
dolomitic structures were originally aragonite crystal fans (ACF).
Variations of stromatolite morphology and frequent occurrences of
storm-related deposits in the Huaiyincun Formation point to repeated cycles
of sea level changes. The bedding-parallel distribution of the ACF and the
homogeneous δ13C values of the ACF-bearing dolostones are
consistent with a primary depositional origin for the ACF. An updated
compilation of published records of ACF throughout geological history
highlights a clear absence of ACF from the initiation of the
Paleoproterozoic Great Oxidation Event until the end of the LJE, and a
global reappearance of ACF in the post-LJE late Paleoproterozoic. We
propose that the reappearance of ACF is in agreement with the expansion of
the oceanic dissolved inorganic carbon reservoir. At the same time,
consumption of dissolved oxygen during the oxidation of organic matter
might have been stimulated by ferruginous deep seawater, facilitating the
formation of Huiayincun ACF.
Large-scale Permo-Triassic back-arc extensions of the
Mongol-Okhotsk Ocean
Mingshuai Zhu; John Wakabayashi; Daniel Pastor-Galán; Fuqin Zhang;
Ariuntsetseg Ganbat ...
The Late Paleozoic−Mesozoic Mongol-Okhotsk orogenic belt marks the final
aggregation of East Asia. The geodynamics of the Mongol-Okhotsk oceanic
plate subduction are still poorly understood due to its curved orogenic
architecture, complex kinematics, and the protracted active continent
margin that developed during oceanic subduction. Here, we report the
discovery of an ophiolite within the southern paleo-active margin of the
Mongol-Okhotsk orogenic belt. The ophiolitic slices are composed of
serpentinites, metagabbros, and metabasalts, and interleaved with deformed
volcaniclastic rocks. Using zircon U-Pb dating, we determined an age of 253
± 2 Ma for the metagabbro, which suggests that the ophiolite formed during
the Late Permian. Geochemical data and geologic relationships indicate that
this ophiolite formed in a back-arc setting. The contemporaneous granitic
dikes (ca. 255 Ma) intruding the basalts of the ophiolite were likely
formed by crustal anatexis during the back-arc extension. Detrital zircon
in sandstone associated with the ophiolite shows a prominent peak age of
ca. 273 Ma that probably represents the sediments of the back-arc basin.
Combining our discovery with the available data from the literature, we
suggest that a >5000-km-long back-arc extension belt existed in the
southwestern segment of the Mongol-Okhotsk belt, which indicates a probable
Western Pacific-type active margin rather than the previously proposed
Andean-type margin that formed during the Permo-Triassic period.
New geochronological results from late Mesoproterozoic to early
Neoproterozoic successions in the eastern North China Craton and
implications for the reconstruction of Rodinia
Chaohui Liu; Guochun Zhao; Fulai Liu; Wang Xu; Xun Sun
Late Stenian to Tonian stratum of the Xihe and Jinxian groups, the Sangwon
system, the Penglai Group, the Tumen Group, and the Huaibei and Langan
groups located at the eastern margin of the North China Craton provide an
excellent record of changes in sediment provenance related to the
amalgamation and dispersal of Rodinia and therefore the paleogeographic
position of the craton. To decipher their protosource changes over time, we
evaluated 7510 U-Pb and 963 Lu-Hf analyses from 92 samples across the
entire eastern North China Craton, of which 1746 U-Pb and 415 Lu-Hf
analyses from 18 samples are newly reported here. The detrital zircon
results indicate two stratigraphic intervals with internally consistent
lithostratigraphy, event stratigraphy, biostratigraphy, depositional age,
and detrital zircon age patterns: the late Stenian to early Tonian strata
and the late Tonian strata, of which the former mainly consists of the
Mesoproterozoic detrital zircons with juvenile Hf isotopic features and the
latter’s detritus chiefly derived from the Neoarchean to late
Paleoproterozoic basement of the North China Craton. In consideration of
the paleomagnetic and geochronological data from the coeval strata in other
cratons, the Mesoproterozoic detrital zircons of the late Stenian to early
Tonian strata are interpreted to be derived from the Musgrave Province in
central Australia during the Rodinia amalgamation, and the transition to
the autochthonic protosources in the late Tonian possibly indicated the
breakup of the North China Craton and North Australian Craton connection.
Relative importance of magmatic and hydrothermal processes for
economic Nb-Ta-W-Sn mineralization in a peraluminous granite
system: The Zhaojinggou rare-metal deposit, northern China
Lu Zhang; Shao-Yong Jiang; Rolf L. Romer; Hui-Min Su
Highly evolved granitic melts typically experience late-stage melt-melt and
fluid-melt immiscibility as well as fluid-melt and fluid-rock interaction.
These processes are particularly important in the formation of deposits of
the rare metals Nb, Ta, W, and Sn. We document the relation between
immiscibility and alteration processes and the partitioning behavior of
rare metals for the Zhaojinggou rare-metal deposit of northern China. This
deposit shows a systematic change from a magmatic to a hydrothermal system,
including the reaction of the exsolved fluid with earlier crystallized
granite and the formation of late-stage quartz veins. The magmatic stage
(Stage I) includes biotite alkali-feldspar granite (BAG) with moderate
Nb-Ta mineralization. Extreme fractional crystallization of BAG eventually
resulted in melt-melt immiscibility and the separation of a hydrosaline
melt. Fractional crystallization of this hydrosaline albite granite (AG)
melt finally exsolved a magmatic fluid. Therefore, the
magmatic-hydrothermal transition (Stage II) includes a melt-dominated Stage
IIa with strong Nb-Ta-Sn mineralization in AG and a fluid-dominated Stage
IIb with minor Nb-Ta-Sn mineralization in muscovite and biotite greisen.
Late hydrothermal processes (Stage III) formed quartz veins with important
W mineralization. There are several texturally and chemically distinct
generations of cassiterite and columbite-group minerals (CGM) in BAG and AG
reflecting crystallization from an evolving magma. The porous and
patchy-zoned reaction rims of tantalite-(Mn) and wodginite on CGM in AG are
the result of fluid-melt interaction. Texture and compositions show that
wolframite in AG is hydrothermal and formed through interaction of early
exsolved magmatic fluids with the host granite. CGM and cassiterite in the
biotite greisen and Ta-rutile in the muscovite greisen, as well as
wolframite and scheelite in quartz veins that formed when fluid-rock
interaction reduced the availability of H+ or F− or
the temperature of the fluid decreased. The distribution and importance of
mineralization demonstrate that Nb, Ta, W, and Sn strongly partitioned into
the hydrosaline melt during melt-melt immiscibility and that W partitioned
into the magmatic fluid during fluid-melt immiscibility. Exsolved magmatic
fluids interacted with earlier crystallized rocks mobilizing rare (Nb, Ta,
and Sn) and base (Fe and Ti) metals from Li-Fe mica, providing the ore
elements for subordinate Nb-Ta-Sn mineralization in AG and in biotite and
muscovite greisen. Thus, magmatic processes (with later metal
redistribution by magmatic fluids) dominantly control economic Nb-Ta-Sn
mineralization, whereas hydrothermal processes mainly control the formation
of economic W mineralization.
The good, the bad, and the ugly: Analysis of three arguments in the
ongoing debate concerning the polarity of Mesozoic arcs along the
western margin of North America
Grant W. Lowey
The debate concerning eastward versus westward subduction along the western
margin of Mesozoic North America involves three main arguments: The
tomography argument (westward) claims subducted slabs are observed by
tomographic processing of seismic waves; the geologic evidence argument
(eastward) claims subduction polarity is recorded by tripartite
accretionary complex−forearc basin−magmatic arc assemblages; and the
crucial geologic test argument (eastward) claims endemic terranes are
characterized by detrital zircons of appropriate age and abundance,
particularly Precambrian zircons. Reconstruction of the arguments in
standard logic form as categorical syllogisms indicates that all three
arguments are valid. However, evaluation of the truth-falsehood of the
propositions supporting the arguments suggests: (1) propositions for the
tomography argument seem more truthful than false; (2) propositions for the
geologic test argument seem more false than truthful; and (3) propositions
for the crucial geologic test argument also seem more false than truthful.
Thus, the tomography argument appears to be a “good” argument (i.e., valid
and sound); the geologic evidence argument appears to be a “bad” argument
(i.e., valid and unsound); and the crucial geologic test argument appears
to be an “ugly” argument—in the spirit that some mathematical proofs are
ugly (i.e., valid and unsound, in addition to being convoluted and couched
in vague terms such as “appropriate”). Proposed tests for falsification of
the arguments include application of an alternate tomographic processing
method to obtain non-wall-like structures in the mantle, demonstration that
tripartite successions are not consanguineous, and procurement of
additional evidence showing that endemic terranes do not contain
Precambrian detrital zircons of appropriate age and abundance.
Tracing the protoliths of the garnet amphibolitic and retrogressed
eclogitic slices and a conceptual tectonic model for their
emplacement onto the Central Menderes Massif, Turkey: New
geochemical data and laser ablation-inductively coupled plasma-mass
spectrometry U-Pb zircon and rutile ages
Semih Gürsu; Andreas Möller; Jeremy D. Inglis; M. Cemal Göncüoğlu; Kevin
Hefferan ...
The age, emplacement, and metamorphic history of the garnet amphibolitic
and retrogressed eclogitic slices in the Menderes Massif (Eastern Aegean)
have been a matter of debate since the 1990’s. Late Cretaceous garnet
amphibolitic and retrogressed eclogitic slices in the Bozdağ and Çine
nappes show low-angle tectonic contacts with the surrounding Early Cambrian
meta-siliciclastics in the Alaşehir-Yahyaalcı and late Neoproterozoic
basement rocks in the Birgi and Tire-Çamlıca Klippe in the northern
Menderes Massif. Lower and upper intercept ages on the discordia diagrams
of the garnet amphibolitic slices are 82 ± 230 Ma and 554 ± 14 Ma,
respectively, in the Yahyaalcı-Alaşehir area of the Bozdağ Nappe; and 96 ±
260 Ma and 550 ± 13 Ma, respectively, in the Camlica-Tire Klippe area of
the Çine Nappe. Retrogressed eclogitic slices give dates of 81 ± 13 Ma
lower intercept and 546 ± 13 Ma upper intercept discordant ages in the
Yenişehir-Kiraz area of the Çine Nappe. Lower intercept age of 81 ± 13 Ma
is supported by two spots dated on metamorphic zircons showing positive Eu
patterns yielding 86.0 ± 1.3 Ma concordant age. Zircon U-Pb dating on the
oscillatory zoning spots are dated as 537.5 ± 1.6 Ma (mean square weighted
deviation [MSWD] = 1.5, n = 41) for the garnet amphibolic slices in the
Yahyaalcı-Alaşehir area (Bozdağ Nappe), 539.0 ± 1.1 Ma (MSWD = 1.4, n = 56)
for the garnet amphibolite slices in the Camlica-Tire Klippe area (Çine
Nappe), and 536.6 ± 2.3 Ma (MSWD = 1.7, n = 32) for the retrogressed
eclogitic slices (Çine Nappe). These ages combined with Eu negative
anomalies on the dated spots are evaluated as representing their magmatic
crystallization ages of the protoliths. Garnet isopleth intersections and
pseudosections gave pressure-temperature conditions of 14 kbar and ∼680 °C
for the retrogressed eclogites, whereas garnet amphibolites display
disequilibrium in whole rock scale. We propose that the age of their
protoliths likely correlate with the Cadomian rift-related Early Cambrian
meta-mafic dykes that were subducted beneath the İzmir-Ankara-Erzincan
Neotethyan lithosphere during the Late Cretaceous. The new rutile U-Pb age
of 30.1 ± 2.0 Ma supports that the tectonic slices of the İzmir-Ankara
oceanic lithosphere and metamorphosed Tauride-Anatolide continental margin
were emplaced onto the Menderes Massif to generate the “main Menderes
metamorphic terrane” during the latest Paleocene and early Eocene.
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