New GSA Bulletin Articles Published Ahead of Print in August
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; Atlantic Canada;
the New Caledonian arc; the Farallon plate; and the Hikurangi subduction
margin, New Zealand. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent.
Unconformity-bounded rift sequences in Terreneuvian‒Miaolingian strata
of the Caledonian Highlands, Atlantic Canada
J. Javier Álvaro; Susan C. Johnson; Sandra M. Barr; Sören Jensen; Teodoro
Palacios ...
Abstract:
The Cambrian syn-rift strata preserved in western Avalonia provide a
distinctive example of how unconformity-bounded sequences are diachronous
throughout proximal to marginal rift branches. Terreneuvian‒Miaolingian
third-order sequences of the Caledonian Highlands, New Brunswick, Canada,
reflect a complex interplay among syn-rift tectonic events, denudation
pulses, and sea-level fluctuations. Unconformably overlying the early,
rift-related volcanosedimentary Coldbrook Group (ca. 560‒550 Ma), the
Ratcliffe Brook, Glen Falls, Hanford Brook, and Forest Hills Formations can
be subdivided into two transgressive systems tract (TST)‒highstand systems
tract (HST) sequences (each ∼10 m.y.) and an incomplete TST sequence that
are separated by stratigraphic gaps. They reflect uplift and tilting events
affecting the basement, transgressive and drowning surfaces, and condensed
sections. Arid to semi-arid climatic episodes are supported by the
excellent preservation of mafic to felsic volcanic clasts in non-marine
breccias and conglomerates, which are derived from the Ediacaran basement,
and the local precipitation of marine gypsum through the evaporation of
pore fluids. Early Miaolingian episodes of microbial/shelly carbonate
production preserved precipitates of coeval evaporite (gypsum pseudomorphs
after drusy mosaics of calcite) and ikaite (glendonitic, star-shaped
aggregates and crusts). Both minerals, traditionally considered to be
indicators of contrasting climate conditions, potentially co-occur in
temperate-water substrates recording high rates of microbial activity. The
early rift phases preserved in the western Avalonian rift transect comprise
stepwise uplift and unroofing of rift shoulders, which are related to
diachronous, angular discordances and paraconformities bounded by
syntectonic slope-apron deposits. Facies homogenization was attained during
Miaolingian times as a result of generalized flooding, sealing of
paleotopographies, and blanketing with monotonous offshore-dominant shales.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36402.1/616528/Unconformity-bounded-rift-sequences-in
Fault zone architecture and lithology-dependent deformation mechanisms
of the Himalayan frontal fold-thrust belt: Insights from the Nahan
Thrust, India
Dyuti Prakash Sarkar; Jun-ichi Ando; Gautam Ghosh; Kaushik Das; Prabir
Dasgupta ...
Abstract:
Brittle shallow crustal faults typically develop a complex fault zone
architecture with distinct structural domains that display diverse
microstructures, mineralogy, and deformation mechanisms. The development of
such domains is typically controlled by the strength and composition of the
protoliths, physical conditions of deformation, fluid ingress, and
diachronous fault growth in response to stress accumulation and co-seismic
slip. Herein, we studied the microstructure-mineralogy-kinematics of fault
rocks in the Nahan Thrust, in the vicinity of the Main Frontal Thrust that
represents a tectonically active zone in the Himalayan orogen. The Nahan
Thrust is characterized by alternating red and gray gouge layers, and a
single black gouge layer. Our results from electron microscopy and X-ray
diffractometry indicate that the protolith of the red gouge layers is
argillaceous sandstone, whereas that of the gray and black gouge layers is
sandstone. Microstructures suggest an initially distributed deformation
(aseismic creep), followed by a protracted brittle deformation event, and a
later aseismic creep stage. The brittle stage is marked by progressive
localization of stress, fracture development, cataclasis, frictional
sliding, and seismic slips. The black gouge layer acted as the principal
slip zone and exhibited ultrafine bands of micrometer-scale slip zones with
vapor escape structures and clay clast aggregates, indicating seismic
faulting and frictional heating during seismic slips. The preferential
seismic rupture nucleation in the black gouge layer indicates a strong
lithological dependence on seismic slip in the Nahan Thrust. We also
conclude that heterogeneity within the Nahan Thrust resulted from primary
lithological variations of the protoliths.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36246.1/616520/Fault-zone-architecture-and-lithology-dependent
Constraint on the temperature of A-type magma from contact metamorphic
aureole, Biesituobie batholith, West Junggar in NW China, Central Asian
Orogenic Belt
Yichao Chen; Jiahui Liu; Renjie Zhou; Wenjiao Xiao; Ji’en Zhang ...
Abstract:
The Biesituobie A-type batholith in West Junggar in NW China of the Central
Asian Orogenic Belt contains metapelite xenoliths derived from the contact
metamorphic aureole. These xenoliths could be divided into two types:
biotite-muscovite-andalusite hornfels and cordierite-alkaline
feldspar-sillimanite-corundum gneiss, indicating prograde metamorphism. The
phase equilibrium modeling on the cordierite-alkaline
feldspar-sillimanite-corundum gneiss sample yielded a peak condition of P = 3.0−4.0 kbar, T = 760−800 °C. Similarly, the
Na-in-cordierite geothermometer yielded a peak condition of T =
771−780 °C. The temperature of the peak metamorphism could be considered as
a lower limit of the temperature of the Biesituobie A-type batholith. On
the contrary, the Ti-in-zircon geothermometer applied to the Biesituobie
A-type batholith zircons yields a mean temperature of 672 ± 22 °C. It
suggested that the application of Ti-in-zircon geothermometer on A-type
magma may involve a >100 °C underestimation. Zircon U-Pb dating
indicates that the age of the contact metamorphism is between 263 and 286
Ma, consistent with the age of the pluton at 274 Ma. The result of 760−800
°C calculated from the contact metamorphic aureole of the Biesituobie
batholith put a lower limit on considering the temperature condition of the
A-type granite series from a metamorphic constraint.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36541.1/616521/Constraint-on-the-temperature-of-A-type-magma-from
A case of Ampferer-type subduction beneath the New Caledonia arc:
Evidence for inefficient subduction of hydrated lithologies into the
upper mantle
Kirsten N. Nicholson; Mohamed Ali Abu El-Rus
Abstract:
New Caledonia is one of the world’s best-exposed subduction/obduction
complexes and is central to understanding the geodynamic evolution of the
southwest Pacific region. We present new geochemical and Ar/Ar age dates
from the in situ eruptive sequences of the La Conception basaltic-andesite
lavas (ca. 29.12 Ma) and correlate the generation of these lavas with the
generation of the Saint Louis and Koum/Borindi Massifs (ca. 24 Ma) to
provide information on the magmatic processes operating within the mantle
wedge over time and to indicate the direction of the subducted slab during
the Oligocene. The La Conception basaltic-andesite lavas were emplaced in
an arc-to-trench direction to the southwest of New Caledonia due to the
partial melting of metasomatized, amphibole-bearing garnet peridotites at
the base of mantle wedge (∼112−118 km). However, both the Saint Louis and
Koum/Borindi granodiorite massifs were derived from melting within the
lower crust of an island arc overlying a mantle wedge. Such temporal and
spatial variations of magmatism in New Caledonia are consistent with a
northeast subduction zone during the Oligocene. The absence of voluminous
arc magmatism related to the Oligocene subduction is a consequence of the
low Tp temperatures of the mantle wedge, the old age
and the high sinking velocity of the downgoing slab, and the absence of
stress within the overriding plate. The Oligocene subduction beneath New
Caledonia, therefore, is an ideal example of Ampferer-type subduction,
which is an amagmatic closure area due to the inefficient subduction of
hydrated lithologies into the convective upper mantle. Once the Paleocene
to Miocene contraction in the southwest Pacific region ceased, the
subduction west of New Caledonia shut down, the system relaxed, and no more
melting occurred during the Oligocene.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36166.1/616491/A-case-of-Ampferer-type-subduction-beneath-the-New
Intensified and apace bauxitization over the paleo-karstic surface
linked to volcanism
Xuefei Liu; Qingfei Wang; Yongbo Peng; Runsheng Yin; Yao Ma ...
Abstract:
Recent studies indicate that volcanism may be a potentially important
factor in the formation of karstic bauxite, but more evidence is needed.
Six billion tons of bauxite formed upon paleo-karstic terrain within a
short time in the Late Carboniferous in the North China Basin (NCB) and in
the Late Permian in the Youjiang Basin (YB) of China. The factors that
trigger their apace formation remain unclear. Herein, we proposed that
extensive volcanic eruptions have accounted for this large-scale bauxite
formation based on the mineralogy, Hg isotopes, and Hg concentration
enrichment proxies of the samples of bauxitic sequence in the NCB and YB.
NCB bauxite generally has three layers, namely bottom Fe-bearing claystone,
intermediate bauxite ore, and top claystone, while bottom Fe-bearing
claystone is usually absent in YB bauxite, which directly covers carbonate
rocks. The mineral assemblages of NCB and YB bauxite confirm that they were
deposited in a superficial alkaline and reducing karstic environment.
Strong Hg enrichment peaks with corresponding near-zero ∆199Hg,
a signal of excess volcanogenic Hg, were discovered in the upper claystone
of NCB bauxitic sequences, which overlaid the Ordovician carbonate
substrate. It denotes volcanism occurred immediately subsequent to
deposition of terrestrial weathered Al-rich remnants in paleo-karstic
depressions during the Late Carboniferous. This volcanism, occurring in the
northern margin of the NCB, is considered to have triggered the apace
bauxitization in the NCB after its long exposure and weathering. In the YB,
the remarkable Hg enrichment and near-zero ∆199Hg were observed
in the entire Late Permian bauxitic sequences with Late Permian carbonate
as the substrate. It denotes enhanced volcanogenic Hg inputs throughout the
whole deposition process of bauxite during the Late Permian. This episode
of volcanism associated with the Emeishan large igneous province and
contemporaneous arc system resulted in the short-term weathering of source
materials and the apace bauxitization in YB. Volcanism is suggested to
aggravate surface acidic and oxidizing conditions, in which rapid
decomposition of source materials occurred to release Al3+ to
precipitate diaspore above the physiochemical barrier of underlying
carbonate.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36507.1/616492/Intensified-and-apace-bauxitization-over-the-paleo
Thermal equation of state for zoisite: Implications for the
transportation of water into the upper mantle and the high-velocity
anomaly in the Farallon plate
Shijie Huang; Zhilin Ye; Dawei Fan; Jingui Xu; Dongzhou Zhang ...
Abstract:
Seismic tomography studies have revealed a high-velocity anomaly at depths
between 100 km and 300 km in the Farallon plate. However, the reasons for
the high-velocity anomaly continue to be debated. An analysis of the
mineral proportions of eclogites exhumed from the Farallon plate shows that
the average amount of zoisite in eclogite is ∼16.0 vol%. Therefore, the
presence of zoisite eclogite needs to be considered to explain the
high-velocity anomaly of the Farallon plate. However, the thermal equation
of state and stability of zoisite have not been fully investigated under
high pressure−temperature (P-T) conditions. We
investigated the high-pressure and high-temperature behavior of natural
zoisite utilizing synchrotron single crystal-X-ray diffraction (XRD). The
results indicate that zoisite is metastable up to 24.8 GPa and 700 K. We
obtained the ambient unit-cell volume V0 = 901.26(3) Å 3 by synchrotron single crystal-XRD measurement. We also fitted
the pressure-volume-temperature data to a high-temperature Birch-Murnaghan
equation of state and obtained the zero-pressure bulk modulus K0 = 134.7(8) GPa, the temperature derivative of the
bulk modulus (∂K/∂T)P = −0.011(4) GPa/K, and the
thermal expansion coefficients α0V = 1.9(7) × 10−5 K−1 and α1V = 3(2) × 10−8 K−2 when the pressure derivative of bulk modulus ( K′0) is fixed at 4. By incorporating the results from
previous studies, we calculated the density and bulk sound velocity
profiles of zoisite eclogite along the Farallon plate geotherm. Finally, we
infer that zoisite could carry water to depths of ∼300 km within cold
subducting slabs and that the bulk sound velocity of typical zoisite
eclogite with ∼61.0 vol% omphacite, ∼23.0 vol% garnet, and ∼16.0 vol%
zoisite could cause the high-velocity anomalies at depths of 100−300 km in
the Farallon plate.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36479.1/616493/Thermal-equation-of-state-for-zoisite-Implications
New insights into the source of gold in the Youjiang basin, SW China
Jun Chen; Li-Juan Du; Rui-Dong Yang; Mei-Fu Zhou; Chun-Kit Lai ...
Abstract:
Mantle plume rich in gold is considered to be important for the formation
of giant epigenetic gold deposits. The Youjiang basin, SW China, is the
world’s second largest Carlin-type gold province, but the ultimate source
of its gold remains enigmatic. In this study, we report that the
Middle−Late Permian basaltic rocks in the basin are rich in native gold
grains. These gold grains are scattered in the interstices of pyrite and
marcasite and in the amorphous silica cavities. Mineralogy and S-Pb isotope
geochemistry of the auriferous sulfides suggest that the gold was largely
derived from the gold-rich Late Permian (ca. 260 Ma) Emeishan plume and was
released to a near-surface volcanogenic massive sulfide (VMS) metallogenic
system, where it accumulated. The native gold grains from the basalts may
have been inherited by the younger (ca. 140 Ma) Carlin-type ores in the
Youjiang basin, which are indicative of gold pre-enrichment in the basin.
Our study highlights that golden plume upwelling could carry abundant gold
into the upper crust, even into shallow-level metallogenic systems, and
thus provides an alternative view on the source of gold in the Youjiang
basin.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36520.1/616388/New-insights-into-the-source-of-gold-in-the
Covering the Great Unconformity in southern Laurentia during Rodinia
breakup: Detrital zircon studies of provenance evolution during
Cambrian marine transgression (Llano Uplift, Texas)
J. Richard Kyle; Daniel F. Stockli; Earle F. McBride; Brent A. Elliott
Abstract:
The Llano Uplift in central Texas, USA, exposes the southernmost expanse of
Laurentian crystalline basement in North America and the overlying lower
Paleozoic strata deposited on the Great Unconformity. Systematic detrital
zircon (DZ) U-Pb provenance analysis of the Hickory Sandstone—the basal
unit of the Cambrian Riley Formation that onlaps the Mesoproterozoic core
of the Llano Uplift—yielded locally variable DZ U-Pb signatures with ages
ranging from 1800 Ma to 485 Ma (n >1700). The Hickory Sandstone zircons
are dominated by 1550−1300 Ma (50%) and 1300−1000 Ma (46%) ages. These two
dominant Mesoproterozoic DZ age components likely were sourced from local
Grenville metamorphic and igneous basement of the Llano Province (1300−1000
Ma) and from the extra-regional Granite-Rhyolite Province basement to the
north and northwest of the Grenville Front, which suggests both local
sourcing and a regional drainage system supplying sediment to the southern
Laurentian margin during the Cambrian. This interpretation is supported by
a minor component of 1800−1600 Ma zircons, sourced from the
Yavapai-Mazatzal Province, and zircons with Early Cambrian ages that were
likely sourced from the Wichita Igneous Province, which is located to the
northwest and to the north of the Llano Uplift, respectively. Documented
NNW-trending topographic ridges in the Mesoproterozoic basement surface of
the Llano Uplift could have funneled aeolian and fluvial sand supply to the
southern Laurentian coast that was reworked in the marginal marine
environment. Samples from the western Llano Uplift are dominated by
regionally sourced, early Mesoproterozoic Granite-Rhyolite Province DZ
ages, whereas samples from the eastern Llano Uplift exhibit a dominant
locally sourced Grenville signature. Hickory Sandstone samples also contain
a small number of Neoproterozoic (850−600 Ma) and Cambrian (541−487 Ma)
zircons. Sources for Neoproterozoic zircons likely were located along the
Laurentian continental margins and derived from extension-related magmatism
associated with the breakup of Rodinia. Cambrian zircons are most common in
Hickory Sandstone samples on the southern and western flanks of the Llano
Uplift. Some are reasonably sourced from the Wichita Igneous Province to
the north, but the younger Cambrian zircons suggest sources to the west.
Upper Hickory strata are dominated (>60%) by 1550−1300 Ma grains with up
to 10% >1600 Ma Paleoproterozoic grains. Upper Cambrian sandstones of
the Wilberns Formation also contain a significant contribution of >1600
Ma grains, which suggests a possible enlargement of the fluvial headwaters
over time extending into older Laurentian provinces to the north and
northwest. Differences among DZ populations over relatively limited
distances may have been related to NW-oriented ridge and corridor landforms
that developed on the Great Unconformity surface, which were products of
the Proterozoic tectonic and lithologic architecture affected by aeolian
and fluvial processes at this unusual stage in Earth history. Paleocurrent
data and atypical detrital grain characteristics for a Hickory Sandstone
locality in the southeastern Llano Uplift and its DZ age similarities with
Cambrian sandstones of the Argentine Precordillera suggest a common
provenance prior to or during Rodinia breakup.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36389.1/616389/Covering-the-Great-Unconformity-in-southern
Initial opening of the Neotethyan Ocean in SE Asia: Constraints from
Triassic magmatism and sedimentation
Shili Peng; Touping Peng; Weiming Fan; Guochun Zhao; Xiaohan Dong ...
Abstract:
How the Neotethyan Ocean evolved and extended southwards into Southeast
Asia remains controversial. The paleographical correlation between India
and the SE Asian blocks and/or terranes before the opening of the Neotethys
and the initial opening time of the Neotethys is still unknown. The lack of
this knowledge hampers our further understanding of the tectonic evolution
of global Neotethys. Here we present a combined study on Triassic magmatism
in the Tengchong Block and Triassic sedimentation in the Myitkyina area,
located on the two sides of the Tagaung-Myitkyina Ophiolite Belt of
northern Myanmar. Our results coupled with previous data demonstrate that a
Triassic continental magmatic arc developed in the Tengchong Block and that
the Triassic Myitkyina sedimentary sequence was part of the Tethyan
Himalayan Langjiexue Group in northern India. Moreover, the Tengchong
Triassic magmatic arc provided important detrital inputs to the whole
Langjiexue Group of northern India. Such a provenance is the best
explanation for the Permian−Triassic detrital zircons of the Langjuexue
Group. Together, we propose that the Tagaung-Myitkyina Ophiolites in
northern Myanmar are the relics of the Neotethyan Ocean rather than the
Mesotethyan Ocean (Bangong-Nujiang Ocean) in SE Asia, and that the initial
opening time of the Neotethys was the Early Jurassic of 200−190 Ma. Then,
the earliest (185−165 Ma) intra-oceanic arc akin to the Izu-Bonin-Mariana
arc in the West Pacific, developed soon after the Late Triassic opening of
the Neotethys.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36431.1/616390/Initial-opening-of-the-Neotethyan-Ocean-in-SE-Asia
Reworking of Yangtze crust into the mantle lithosphere of the North
China Craton along the Dabie−East Qinling Orogen: Evidence from the
Early Cretaceous volcanic rocks
Feifei Zhang; Yifan Wang; Peter A. Cawood; Yunpeng Dong; Chenghui Xin
Abstract:
Mineral chemistry, zircon U-Pb geochronology, and elemental and
Sr-Nd-Pb-Hf-O isotopic data for the Early Cretaceous volcanic rocks from
the Dabie and East Qinling orogens of China constrain the reworking history
of the Yangtze crustal materials in the North China lithosphere. These data
provide new insights into the recycling of the deep-subducted crustal
materials into the mantle and the transformation process from continental
subduction to collision. Our data show that the Early Cretaceous volcanic
rocks are characterized by shoshonitic and high-K calc-alkaline basaltic
trachyandesite, trachyandesite, and trachyte. They synchronously erupted at
ca. 135−120 Ma, and have zircon in situ εHf(t) values ranging
from −29.0 to −17.2, and δ18O values from 4.89‰ to 6.84‰. These
samples share similar “crust-like” geochemical signatures, whole-rock
enrichment in the large-ion lithophile elements, depletion in high field
strength elements (Nb/La = 0.12−0.38), highly enriched Sr-Nd isotopic
compositions, as well as (206Pb/204Pb)i =
15.97−17.59, (207Pb/204Pb)i = 15.33−15.54,
and (208Pb/204Pb)i = 36.87−38.59. The ε Nd(t) values range from −24.4 to −15.6, −21.8 to −13.1, and
−16.8 to −10.9 for the eastern and western North Huaiyang belts of the
Dabie Orogen and the East Qinling Orogen, respectively. Such geochemical
similarities suggest that the northward deep-subduction of Yangtze crustal
materials have been reworked into the North China Craton enriched
lithospheric mantle. The spatial pattern of the Early Cretaceous volcanic
rocks suggests that the Yangtze subduction extended northward beyond the
southern margin of the North China Craton and ran into its interior. The
surface and deep lithospheric boundaries are decoupled between the Yangtze
Block and North China Craton. The Early Cretaceous igneous rocks in the
Dabie−East Qinling Orogen were induced in response to the post-collisional
unrooting of the Triassic over-thickened lithosphere coupling.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36395.1/616315/Reworking-of-Yangtze-crust-into-the-mantle
A combination of plume and subduction tectonics contributing to breakup
of northern Rodinia: Constraints from the Neoproterozoic magmatism in
the Dunhuang-Alxa Block, northwest China
Rongguo Zheng; Jinyi Li; Wenjiao Xiao; Jin Zhang
Abstract:
Neoproterozoic igneous rocks are widely distributed in the Dunhuang-Alxa
Block, northwest China, and record geodynamic processes caused by the
assembly and break-up of Rodinia. In this study, we present new
petrological, geochemical, and zircon U-Pb-Hf isotope data for the Langshan
gabbros in the northeastern Alxa Block. Langshan gabbros (827−819 Ma) have
depleted whole-rock Nd (+3.2 to +4.9) and zircon Hf (+4 to +21) isotopic
compositions and weak enrichments in large-ion lithophile elements. These
rocks were probably derived by partial melting of a depleted mantle wedge
metasomatized by fluids released from a subducted slab in a back-arc basin
environment. Geochemical and zircon Hf isotope data for early
Neoproterozoic igneous rocks are consistent with the presence of a
long-lived (ca. 930−810 Ma) retreating subduction zone along the northern
margin of the Dunhuang-Alxa Block, indicating it was located at the
northern margin of Rodinia. Some mantle plume-related magmatism, including
A1-type silicic volcanic rocks in the Langshan Group and
Jinchuan ultramafic-mafic intrusions, occurred in the interior of the
Dunhuang-Alxa Block, which was similar to early Neoproterozoic
plume-related magmatism (ca. 850−820 Ma) in the interior of northern
Rodinia. The temporal link between subduction and extension suggests that
this retreating subduction zone along the northern margin of Rodinia, in
combination with a mantle plume beneath its interior, resulted in
lithospheric extension and break-up of Rodinia during ca. 850−810 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36416.1/616316/A-combination-of-plume-and-subduction-tectonics
Pressure-temperature evolution of the Qingshuiquan mafic granulite:
Implications for Proto-Tethys subduction in the East Kunlun orogenic
belt, northern Tibetan Plateau
Dengfeng He; Yunpeng Dong; Christoph A. Hauzenberger; Shengsi Sun; Xiaoming
Liu ...
Abstract:
Granulite is in general a key metamorphic rock that can be used to
understand the tectonic architecture and evolutionary history of an
orogenic belt. The Qingshuiquan mafic granulite in the East Kunlun orogenic
belt, northern Tibetan Plateau, occurs as tectonic boudins together with
lower-grade ophiolitic mélange assemblages within an amphibolite-facies
crystalline basement. In this study, we investigated the geochemistry,
geochronology, mineralogy, and phase modeling of the Qingshuiquan mafic
granulite. Based on mineralogical observations and microstructures, three
mineral assemblage generations were distinguished: an assemblage found as
inclusions within garnet and amphibole comprising clinopyroxene +
plagioclase + amphibole + quartz + ilmenite + rutile (M1); an inferred peak
assemblage of garnet + clinopyroxene + plagioclase + amphibole + quartz +
ilmenite ± orthopyroxene (M2) in the matrix; and a retrograde assemblage of
amphibole and biotite coronae (M3) around clinopyroxene or orthopyroxene.
Thermobarometric calculations and phase equilibrium modeling constrained a
clockwise pressure-temperature (P-T) path for the
Qingshuiquan mafic granulite with peak T conditions of 830−860 °C
at 8.0−9.5 kbar. Prior to the peak T conditions, a pressure
maximum of ∼11 kbar at ∼800 °C was recorded by rutile, ilmenite, and
clinopyroxene inclusions in garnet and amphibole. The retrograde path was
defined by a decompression segment followed by final cooling. The
whole-rock geochemical results indicated that the protolith of the
Qingshuiquan mafic granulite was similar to present-day enriched
mid-ocean-ridge basalt (E-MORB) displaying low total rare earth element
(REE) concentrations and a slight enrichment of light REEs, as well as flat
high field strength element patterns in the primitive mantle−normalized
trace-element diagram. Geochronologic results revealed that the protolith
crystallization age of the mafic granulite is 507 ± 3 Ma, and the timing of
granulite-facies metamorphic overprint is 457−455 Ma. This evidence, taken
together with results from previous studies, indicates that the protolith
of the Qingshuiquan mafic granulite can be interpreted as basaltic rocks of
Proto-Tethys oceanic crust that experienced a first high-pressure
granulite-facies imprint followed by subsequent decompression and
granulite-facies overprint at slightly lower P and slightly higher T. This granulite-facies metamorphism can be attributed to the
subduction of Proto-Tethys oceanic crust, which also generated numerous
contemporaneous subduction-related magmatic rocks in the East Kunlun
orogenic belt.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36420.1/616317/Pressure-temperature-evolution-of-the-Qingshuiquan
Reconstructing a dismembered Neogene basin along the active Hikurangi subduction margin, New Zealand
Benjamin R. Hines; Hannu Seebeck; James S. Crampton; Kyle J. Bland; Dominic
P. Strogen
Abstract:
The East Coast Basin (ECB), New Zealand, preserves the most complete
onshore stratigraphic record of the Cretaceous−Neogene development of
Zealandia from Gondwana breakup, through rift-drift, to evolution of the
modern Hikurangi subduction margin and Pacific-Australia plate boundary. As
such, it provides important constraints for southwest Pacific plate
tectonic reconstructions. The basin is, however, deformed and variably
dismembered, and in previous tectonic models it has been treated as a zone
of poorly constrained deformation. Here, multiple geological and
geophysical data sets are integrated with a tectonic reconstruction to
provide a synthesis of regional and intra-basin-scale structural evolution
of the ECB during the Neogene, producing a new geologically constrained
approach toward qualitative and quantitative assessment of deformation
across the New Zealand plate boundary zone. We produce the first
palinspastic reconstructions for the entire basin during key times in plate
boundary development, within a well-constrained plate-tectonic framework,
supported by independent deformation estimates. These reconstructions
account for contraction, strike-slip, and vertical-axis rotation of crustal
blocks. In the context of the reconstruction model, the ECB has dominantly
experienced upper-plate shortening and vertical-axis rotations; strike-slip
processes are considered comparatively minor. Comparison of the
reconstruction model with independent data highlight several pronounced
deformation intervals (26−23 Ma, 20−15 Ma, 11 Ma, and 7−4 Ma) in the
structural and sedimentary evolution of the ECB, Hikurangi subduction
margin, and New Zealand plate boundary zone, demonstrating that it is
possible to integrate a wide range of geological data sets to develop
meaningful reconstructions in highly tectonised regions.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36308.1/616259/Reconstructing-a-dismembered-Neogene-basin-along
The long-lived transcrustal magmatic systems of Southeast China in
response to paleo-Pacific plate subduction, recorded by the Cretaceous
volcanic sequences in southeastern Zhejiang Province
Liang Liu; Jie-Hua Yang; Li-Fang Kang; Hong Zhong; Xing-Chun Zhang
Abstract:
Processes taking place in subduction zones are highly debated. The
Cretaceous volcanic rocks are voluminously distributed along the coastal
area of southeastern (SE) China. To elucidate their petrogenesis and
relationship with subduction, we use new zircon U-Pb ages and Hf-O isotopes
for the representative Cretaceous volcanic sequences in Zhejiang Province,
China. According to stratigraphic investigations, zircon U-Pb ages, and
Hf-O isotopes, these volcanic rocks can be divided into different groups
corresponding to three stages of volcanic activity: the early (Stage 1,
136−129 Ma), middle (Stage 2, 125−115 Ma), and late (Stage 3, 110−94 Ma)
stages. Diverse zircon populations (including antecrysts, autocrysts, and
xenocrysts) are recognized. Hf-O isotopes of autocrysts suggest different
protoliths for the identified three stages of volcanic activity. Xenocrysts
show obvious different compositions from autocrysts. Antecrysts share
similar compositions with autocrysts, which favor similarities in the
magmas from which they were generated. Our observations (very small age
intervals between antecrysts and autocrysts, sources for volcanic rocks
throughout the entire crust, and volcanic magmatism with long-term and
discontinuous characteristics), were inconsistent with the traditional
melt-dominated magma chamber model. Consequently, we propose that those
volcanic rocks were derived from long-lived transcrustal magmatic systems
(TCMS), dominated by crystal mush, instead of melt-dominated magma
chambers, maintained and recharged by a discontinuous contribution of
contemporaneous underplated mantle-derived magmas, triggered by
paleo-Pacific plate subduction. We suggest the different stages of volcanic
activity and corresponding long-lived TCMS were produced by the change of
Pacific plate motion beneath SE China during the Cretaceous period.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36448.1/615998/The-long-lived-transcrustal-magmatic-systems-of
Late Archean−Paleoproterozoic plate tectonics along the northern margin
of the North China craton
Chen Wu; Guosheng Wang; Zhiguang Zhou; Xiaoqi Zhao; Peter J. Haproff
Abstract:
The North China craton of central Asia is a natural laboratory for
investigating early Earth tectonic processes including subduction and
large-scale horizontal crustal motions. However, it remains unclear how and
when the North China craton formed from the amalgamation of several blocks
and orogens including the Archean Western and Eastern blocks, the late
Archean Central Orogenic Belt, and the Paleoproterozoic Northern Margin
orogen. In this study, we integrated new and existing geological field
observations, zircon and baddeleyite U-Pb geochronology, whole-rock
geochemistry, and Sr-Nd-C-O isotope analyses along the northern margin of
the North China craton to improve our understanding of its late
Archean−Paleoproterozoic tectonic evolution. Observations show the Wuchuan
ultramafic-mafic complex of the Northern Margin orogen contains a mixture
of mylonite gneiss, serpentinized ultramafic blocks, and amphibolite
pyroxenite xenoliths. Geochronology and geochemistry results suggest that
the ca. 2.55−2.5 Ga development of the Wuchuan ultramafic-mafic complex was
associated with subduction, which was followed by ca. 2.39 emplacement of
lamprophyre dikes during continental rifting. Subsequent Paleoproterozoic
subduction magmatism, continental collision, and post-orogenic rifting
between the North China craton and southern margin of the Siberian craton
are evidenced by ca. 2.0 Ga arc granitoids, ca. 1.87−1.85 Ga A-type,
garnet-bearing granitoids and mafic dikes, and ca. 1.86 carbonatite. Ca.
2.0 Ga collision and ca. 1.87−1.85 Ga rifting were coincident with
orogen-normal left-slip shear. Our findings show that the ca. 1.9 Ga North
China-Siberia collision was a key event during the assembly of the Columbia
supercontinent that occurred via modern plate-tectonic processes.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36533.1/615927/Late-Archean-Paleoproterozoic-plate-tectonics
Lithospheric mantle provinces and crust-mantle decoupling beneath
northeastern China: Insights from peridotite xenoliths
A-Bing Lin; Sonja Aulbach; Jian-Ping Zheng; Ronghua Cai; Jingao Liu ...
Abstract: The origin and evolution of the subcontinental lithospheric
mantle (SCLM) underlying orogenic belts bordering cratons, as either newly
added domains or strongly reworked older cratonic lithosphere, remains
controversial. This limits our understanding of deep lithospheric behavior
and processes during plate convergence. Here, we use detailed petrology,
whole-rock and mineral compositions, and in-situ Sr isotopic compositions
for spinel-facies peridotite xenoliths from Jiaohe (northeastern China) in
the southeastern Central Asian Orogenic Belt (CAOB), which is pinched
between two NE-striking Mesozoic to Cenozoic giant trans-lithospheric fault
systems. These data are combined with whole-rock Re-Os isotope and
platinum-group element compositions and literature data, to explore the
physico-chemical evolution of the regional SCLM and examine crust-SCLM
relationships for this vast swath of continental lithosphere. Lherzolites
predominate at Jiaohe, and have higher whole-rock Al2O 3 contents than harzburgites (2.27−3.46 wt% versus 0.84−1.02
wt%), but exhibit FeO enrichment similar to harzburgite (bulk FeO up to
9.54 wt%). The lherzolitic clinopyroxenes (Cpx) have higher heavy rare
earth element (HREE) levels (2.79−5.11 ppm) and Ti/Eu (3882−6864), coupled
with wider variation of 87Sr/86Sr (0.7021−0.7038) and
lower average ƒO2 (oxygen fugacity relative to the
fayalite-magnetite-quartz buffer; FMQ-1.92 ± 0.55) compared to the
harzburgites (HREE: 0.94−2.11 ppm; Ti/Eu: 163−2044; 87Sr/ 86Sr: 0.7032−0.7036; ƒO2 = FMQ-1.25 ± 0.20). All
these observations suggest that the lherzolites were produced by adding Cpx
± spinel to the protoliths from reducing silicate melts, whereas the
harzburgites may have originated from a remnant, highly depleted mantle
domain that variably interacted with silica-undersaturated oxidizing melts.
Bulk-rock and mineral compositional relationships as well as ƒO2
for peridotite samples from the southeastern CAOB (including Jiaohe) are
similar to those of the northeastern NCC (including Huinan). Their
characteristics contrast with those from the northwestern CAOB, suggesting
that they belong to distinct lithospheric provinces with respect to both
composition and oxidation state. Jiaohe harzburgites yield Paleoproterozoic
Re depletion Os model ages (TRD up to 1.76 Ga), similar to
Huinan peridotites (up to 1.92 Ga), just 170 km away, and may also
represent strongly reworked cratonic mantle. By analogy with evidence from
recent continental collision zones, the Jiaohe SCLM may have originated as
cratonic SCLM which migrated northeastward during plate convergence along
giant trans-lithospheric fault systems.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36338.1/615928/Lithospheric-mantle-provinces-and-crust-mantle
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