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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
GSA BULLETIN articles published ahead of print are online at https://bulletin.geoscienceworld.org/content/early/recent
