Arc-continent Collision Research Articles

Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history

Deep tectonic processes are key integral components in the evolution of mountain belts, while observations of their temporal development are generally obscured by thermal resetting, retrograde alteration and structural overprinting. Here we recorded an integrated rock time-temperature history for the first time in the pro-wedge part of the active Taiwan arc-continent collision starting from sedimentation through cleavage-forming state to its final exhumation. The integrated thermal and age results from the Raman Spectroscopy of Carbonaceous Material (RSCM) method, zircon U-Pb laser ablation dating, and in-situ40Ar/39Ar laser microprobe dating suggest that the basal accretion process was crucial to the development of the Taiwanese orogenic wedge. The basal accretion process commenced early in the mountain building history (∼6 Ma) and gradually migrated to greater depths, as constrained by persistent plate convergence and cleavage formation under nearly isothermal state at similar depths until ∼ 2.5 Ma recorded in the early-accreted units. Such development essentially contributed to mountain root growth by the increased depth of the wedge detachment and the downward wedge thickening during the incipient to full collision stages in the Taiwan mountain belt.

Ca. 850 Ma magmatic events in the Tarim Craton: Age, geochemistry and implications for assembly of Rodinia supercontinent

The origin of magmatic episodes between final assembly and initial breakup of Rodinia is crucial for deciphering geodynamic transition and continental crust growth processes. However, few igneous rocks were identified during the quiescent time from 900 Ma to 830 Ma in the Tarim Craton. Along its northwestern margin, our field investigations identified abundant granitic debris in the Cryogenian Qiaoenbrak Group. Zircon U-Pb age determination revealed crystallization age at ca. 850 Ma. Detrital zircon crystals within Cryogenian to Ediacaran clastic rocks and schists of the basement Aksu Group also contain a major population having crystallized at ca. 860–840 Ma. Thus, we suggest that a hitherto unknown major magmatic event occurred ca. 850 Ma ago in northern Tarim. Granite gravels exhibit typical fractionated granite features, with high values of SiO2 (73.1–75.7 wt%), alkali oxides (Na2O + K2O = 7.8–8.7 wt%) and Na2O/K2O (avg. 1.3 wt%), and low values of P2O5 (≤0.04 wt%) and A/CNK (0.87–1.01), together with Rb, Th, U and K enrichment, and depletion in Nb, Ta, Sr, P, Ti and Eu, negative Eu anomalies (δEu = 0.21–0.74) and fairly low Ga/Al ratios. Including detrital crystals, ca. 850 Ma zircon crystals display a large range of εHf(t) values from −17.0 up to +11.6. Elemental and zircon Hf isotopes suggest that granites were derived from melting of juvenile lower crust, with addition of reworked upper crust and mantle-derived melts. We propose (1) before ca. 850 Ma, arc-continental collision setting related to southward accretionary assembly along Tarim margin, (2) progressive assembly of Tarim Craton, and (3) position of Tarim Craton within Rodinia supercontinent probably closer to North India Block than to Australia at ca. 850 Ma.

Petrogenesis and tectonic implication of Paleoproterozoic granites and granulites in the Fengzhen area of North China craton

The North China Craton (NCC) is one of the oldest cratons in the world that preserves the records of many important geological events in the early evolution of the Earth. However, the early Precambrian tectonic evolution history of the NCC is still controversial. Here we report Paleoproterozoic leucogranites, garnet granites and granulites in the Fengzhen area that constitutes a junction between the Khondalite Belt and the Trans-North China Orogen (TNCO). The leucogranites have high SiO2 content (75–85wt%), and magmatic zircon U-Pb age of 2199±3 Ma. Their zircons have slightly various εHf(t) values (−0.45 to 1.41) with TDM model ages of 2.5–2.6Ga. The granites show strong LREE enrichment with La/Ybcn ratio of 12–176, and have high initial Nd isotope (εNd(t)=3.5–4.5). Most of them display positive Eu anomaly, excluding fractional crystallization contribution on the high Si feature. The leucogranite was produced by melting of Late Archean felsic TTGs (tonalite–trondhjemite–granodiorite) and addition of juvenile crust. Magmatic zircons yield the garnet granite age of 2002±3Ma. The zircons show various εHf(t) values (−5.8 to 7.1) and TDM model ages (2.1–2.6Ga). Low initial εNd(t) isotope value (−4.2 to −8.3) was determined in the granites. The granulites are high pressure metamorphic rocks with a peak pressure of 12.0±0.8 kbar, and have similar metamorphic age of 1967±10Ma to the garnet granites. This indicates that the Fengzhen area underwent 2-Ga inter-continental orogeny, in which the melting of sedimentary rock formed the garnet granites. Similarly, sodic TTG melting to form K-rich leucogranites also needs high pressure. However, the leucogranite age is obviously older than the peak metamorphic timing of continent–continent collision. This implies that the Fengzhen area may have underwent arc-continental collision at 2.2Ga.

Geochronological and petrogenetic constraints on the regional tectonic evolution of the Guanghua Group in northeastern Jiao-Liao-Ji Belt, China

A sequence of Paleoproterozoic meta-supracrustal rocks, known as the “Guanghua Group”, is exposed to the north of Tonghua City, Jilin Province, China. The composition and ages of formation and metamorphism of the protolith, and the tectonic petrogenesis of the Guanghua Group are poorly understood. Comprehending these rocks are important in constraining the formation and evolutionary history of the northern segment of the Paleoproterozoic Jiao-Liao-Ji Belt. We have studied the petrology and geochemistry of these meta-supracrustal rocks, revealing that the protoliths can be sub-divided into two groups: metavolcanic and metasedimentary rocks. The metavolcanic rocks are mainly amphibolite, biotite-bearing plagiogneiss, and minor biotite leptynites, whose protoliths were basaltic andesite, andesite, and minor rhyolite with both calc-alkaline and tholeiite affinities. Petrogenetic studies indicate that the intermediate-basic rocks were derived from a depleted lithospheric mantle wedge that was metasomatized by slab-derived fluids or melts. The metasedimentary rocks consist of biotite–quartz schist and staurolite-bearing garnet-muscovite schist, together with minor biotite-bearing plagiogneiss and biotite leptynites, with protoliths mainly comprising mudstone with subordinate sandstone and greywacke. The structure, composition, and U–Pb ages of zircon indicate that the protoliths of the metavolcanic and metasedimentary rocks formed at ca. 2.18Ga and 2.03Ga, respectively, and were modified by two stages of metamorphism at ca. 1.90 and 1.85Ga. The protoliths of the metasedimentary rocks were derived mainly from mid-Paleoproterozoic granites, with minor contributions from coeval metabasic rocks and Archean granitic rocks. Based on these findings and existing regional data, we suggest that: (i) the Paleoproterozoic supracrustal rocks of the Guanghua Group are comparable with the Laoling Group in their age of formation, composition, and petrogenesis, and were the products of the same geological event; meaning that rocks of the Guanghua Group should be considered as part of the Laoling Group; (ii) the metabasic and metasedimentary rocks formed in a back-arc basin setting; and (iii) the pre-mid-Paleoproterozoic formation and evolution of the northern segment of the Jiao-Liao-Ji Belt were related to arc–continent collision.

In situ zircon U-Pb dating and whole-rock geochemistry of metasedimentary rocks from South Liaohe Group, Jiao-Liao-Ji orogenic belt: Constraints on the depositional and metamorphic ages, and implications for tectonic setting

The Liaohe Group is an important Paleoproterozoic lithostratigraphic unit of the Jiao-Liao-Ji orogenic belt and traditionally subdivided into the North and South Liaohe Groups. However, their tectonic setting and evolution are still a matter of debate. The precise depositional and metamorphic data of the studied metasedimentary rocks from the South Liaohe Group provide important information on the controversial discussion of the tectonic setting and evolution. Based on cathodoluminescence (CL) imaging and connecting LA-ICP-MS U-Pb geochronology, zircon grains separated from metasedimentary rocks of different formations of the South Liaohe Group were subdivided into detrital and metamorphic zircons or zircon domains. Igneous zircon cores of detrital zircons of the Li’eryu, Gaojiayu, Dashiqiao and Gaixian Formations yield U-Pb age peaks at ∼2130 Ma, ∼2520 Ma, ∼2160 Ma/∼2500 Ma, and ∼2040 Ma, indicating that their sediments were transported from the Liaoji Granitoids, the Archean basement rocks of the adjacent Complex. In addition, deposition age of the sedimentary precursors was investigated which must have taken place at some time after ∼2.05 Ga, ∼2.07 Ga, ∼2.04 Ga and ∼1.92 Ga, respectively. These new geochronological data can be well explained by (1) a continent-arc collision model which suggests that the South Liaohe Group formed an intervening island arc and foreland basin sandwiched between the Longgang and Nangrim Complexes, and (2) a northward subduction of the Nangrim Complex source rocks of the South Liaohe Group; these source rocks were successively deposited along the margin of the two Complexes.

Late Neoarchean magmatism and tectonic evolution recorded in the Dengfeng Complex in the southern segment of the Trans-North China Orogen

The Dengfeng Complex exposed in Songshan area in the southern segment of the Trans-North China Orogen (TNCO) mainly consists of Late Neoarchean tonalite-trondhjemite-granodiorite (TTG) gneiss, metadiorite and amphibolite. The Sanhuangzhai amphibolite records the earliest mafic magmatism (2658±5Ma) in the region, which is enriched in LREE and LILE with negative Nb, Ta and Ti anomalies and positive initial εNd(t) and εHf(t) values, suggested to be the remnant of an arc-related juvenile mafic crust. The Shipaihe amphibolite at the end of Neoarchean (2492±7Ma) is characterized by flat REE pattern, slightly negative Nb, Ta and Ti anomalies and positive initial εNd(t) and εHf(t) values, which is interpreted as large-degree partial melt of metasomatised mantle source in a back-arc basin. Zircon U-Pb dating shows two episodes of TTG magmatism (2.57–2.55Ga and 2.55–2.50Ga) within the Dengfeng Complex. All TTG gneisses have high SiO2 and Na2O, but low Mg#, Cr, Ni, [Yb]N and Y with high [La/Yb]N and Sr/Y. The early TTG gneiss (2.57–2.55Ga) in the eastern Songshan area has variable whole-rock Hf-Nd isotopes (εHf(t)=−2.14–4.64; εNd(t)=0.11–3.35; respectively), and its protolith is interpreted as the result of partial melting of a thickened lower crust with residual garnet and amphibole in the source. But the later TTG gneiss (2.55–2.50Ga) in the western Songshan area has depleted whole-rock Hf-Nd isotopes (εHf(t)=5.59–6.08; εNd(t)=2.42–3.86; respectively), suggesting an overall juvenile source, and its protolith is interpreted as the partial melt of flat-subducting oceanic crust. The Shipaihe metadiorite is characterized by high Mg#, Cr, Ni, Sr, Ba, highly fractionated REE patterns and strong negative Nb, Ta and Ti anomalies, resembling to sanukitoid suite, which is interpreted as the partial melt of a mantle wedge metasomatised by subduction-derived fluids/melts. Integrated geochronology and geochemical data of the Dengfeng Complex in Songshan area reveal the Late Neoarchean crustal accretion process from subduction to arc-continental collision (accretionary orogeny) followed by post-collisional extension (back-arc basin), implying an active continental margin arc environment in the southern segment of the North China Craton.

Monitoring interseismic activity on the Ilan Plain (NE Taiwan) using Small Baseline PS-InSAR, GPS and leveling measurements: partitioning from arc-continent collision and backarc extension

Monitoring interseismic activity on the Ilan Plain (NE Taiwan) using Small Baseline PS-InSAR, GPS and leveling measurements: partitioning from arc-continent collision and backarc extension

Origin of the Dongsha Event in the South China Sea

Post-rift tectonic activities have been widely observed in the northern continental margin of the South China Sea, especially during the late Miocene. Large numbers of faults became active. Unconformities, uplift of faulted blocks, sequence tilting, erosion along the Dongsha massif and canyon incision were also discriminated at this stage in the Pearl River Mouth basin (PRMB) and the area to the east. This tectonism has been named Dongsha Event. A number of hypotheses have been put forward to explain the mechanism of the Dongsha Event, such as high-velocity lower crustal flow, magmatic underplating, and arc–continent collision. To investigate the tectonic dynamics, sequence contact relationships, fault activities, and magmatism were analyzed along large numbers of seismic profiles that cover the eastern PRMB and Southwest Taiwan Basin. The timing, affected regions, and differences in the intensity of tectonic deformation were assessed, upon which the plate bending model was favored. In order to check the reasonableness of plate bending model, effective elastic thickness and other geodynamic parameters were calculated constrained by uplift area width and regarding the trench as sediment filling. A maximum Te value of ~27 km and a minimum value of ~4 km were obtained. Integrating with the former stress field calculation, we conclude that the Dongsha Event was mainly affected by subduction and collision of the South China Sea toward the Philippine Sea plate. This event commenced at about 10 Ma and peaked at around 3.6 Ma. Although the high effective elastic thickness required is a problem to be addressed, this research provides by far the most comprehensive evidences to the mechanism of the Dongsha Event.

Kinematics of the tectonic blocks and active faults at the post-orogenic stage: Northern Taiwan

Northern Taiwan offers a rare opportunity to quantify the rate and style of deformation in the transition zone, from the collision in the southeast, subduction, rifting, to orogenic collapsing in the northwest. The updated GPS velocity field collected from 2002 to 2014 and leveling vertical velocity field from 2004 to 2010 were inverted by a 3-D block model in this study to infer the tectonic block rotations and fault slip rates in reference to the Eurasian plate. Our optimized modeling results show that the clockwise rotation rate of 20.8°±2.8°/Myr in the Hualien block is caused by the coupling of the waning arc-continent collision between the Luzon volcanic arc and the Eurasian plate, and the northeastward migration of the block toward the Ryukyu trench. In Ilan domain, the rapid clockwise motion was derived at a rate of 34.9°±65.9°/Myr in southern part of the plain. We proposed that the rapid clockwise rotation results from the asymmetry rifting at the southwestward extension of the Okinawa trough. An another new key finding is highlighted in the capability of maximum magnitude ∼6.2 is indicated on the Suao and the Choshui faults in the Ilan plain.

Rapid post-rift tectonic subsidence events in the Pearl River Mouth Basin, northern South China Sea margin

Data from 26 drill wells and 27 regional seismic profiles were integrated to investigate the timing, phase and origin of the post-rift subsidence in the middle to eastern Pearl River Mouth Basin (PRMB) of the northern South China Sea using the traditional 1-D backstripping technique. Different from previous research of backstripped tectonic subsidence in the basin, we calculated the tectonic subsidence using the newly built local porosity-depth relationships for decompaction and updated sedimentological and paleontological data for paleobathymetry reconstruction. Well-data based subsidence curves reveal a roughly decaying pattern in both the magnitude and rate of the post-rift subsidence in the PRMB, which is in accordance with the general decreasing trend of the thermal subsidence typical of a passive margin. Two events of rapid post-rift tectonic subsidence were identified, which occurred in the Early to early Middle Miocene and the Pliocene. The timing of the first rapid post-rift subsidence event varies and is earlier in the southern rather than northern part of the basin. Additionally, the amplitude of contemporaneous tectonic subsidence is greater in the southern part of the basin. The second rapid tectonic subsidence event occurred simultaneously in both the southern and northern parts of the basin, with the amplitude of subsidence being much greater in the southern part. We associate the first rapid subsidence event with the southward jump of the South China Sea spreading ridge, which occurred between the Oligocene and Early Miocene, while the second event with the arc-continent collision at Taiwan since the latest Late Miocene. The southern PRMB in the deep-water slope area shows a much higher magnitude of tectonic subsidence in both events than its northern counterpart in the shelf area, which could be associated with its much thinner lithosphere. The latter could cause upswelling of denser lower crust and upper mantle material, resulting in more rapid tectonic subsidence in the southern PRMB.

Ultramafic to mafic granulites from the Larsemann Hills, East Antarctica: Geochemistry and tectonic implications

The Larsemann Hills area is part of a reworked early Neoproterozoic metamorphic terrain in southwestern Prydz Bay, East Antarctica. Ultramafic and mafic granulites, whose origins remain controversial, occur as lenses, boudins or layered bodies within the para- and ortho-gneiss in the region. The ultramafic and mafic granulites show spinel-olivine-bearing and two-pyroxene-bearing mineral assemblages recrystallized at 860–900°C. Their bulk rock analyses indicate an origin as igneous cumulates, with high Mg# (molar MgO/(MgO+FeO)) from 0.73 to 0.84 for ultramafic granulite and from 0.46 to 0.78 for mostly mafic granulite as well as high Cr and Ni contents ([Cr] and [Ni] up to 1826ppm and 1400ppm respectively for ultramafic granulite and [Cr] of 1460ppm for mafic granulite). Trace element patterns show pronounced negative Nb anomalies, suggesting a subduction-related tectonic setting for their precursors, consistent with derivation from arc basalts, also suggested by low TiO2 and K-enrichment in mafic granulites. The ultramafic to mafic granulites may have been formed in a subduction-related continental back-arc basin environment simultaneously with peak metamorphism associated with arc-continent collision during the early Neoproterozoic (990–900Ma) Rayner orogeny.

Tropical weathering of the Taconic orogeny as a driver for Ordovician cooling

Research Article| June 09, 2017 Tropical weathering of the Taconic orogeny as a driver for Ordovician cooling Nicholas L. Swanson-Hysell; Nicholas L. Swanson-Hysell 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Search for other works by this author on: GSW Google Scholar Francis A. Macdonald Francis A. Macdonald 2Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Nicholas L. Swanson-Hysell 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Francis A. Macdonald 2Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Publisher: Geological Society of America Received: 23 Jan 2017 Revision Received: 11 Apr 2017 Accepted: 19 Apr 2017 First Online: 23 Jun 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2017 Geological Society of America Geology (2017) 45 (8): 719–722. https://doi.org/10.1130/G38985.1 Article history Received: 23 Jan 2017 Revision Received: 11 Apr 2017 Accepted: 19 Apr 2017 First Online: 23 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Nicholas L. Swanson-Hysell, Francis A. Macdonald; Tropical weathering of the Taconic orogeny as a driver for Ordovician cooling. Geology 2017;; 45 (8): 719–722. doi: https://doi.org/10.1130/G38985.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Earth’s climate cooled through the Ordovician Period leading up to the Hirnantian glaciation. Increased weatherability of silicate rocks associated with topography generated on the Appalachian margin during the Taconic orogeny has been proposed as a mechanism for Ordovician cooling. However, paleogeographic reconstructions typically place the Appalachian margin within the arid subtropics, outside of the warm and wet tropics where chemical weathering rates are highest. In this study, we reanalyze the paleomagnetic database and conclude that Ordovician constraints from cratonic Laurentia are not robust. Instead, we use paleomagnetic data from well-dated volcanic rocks in the accreting terranes to constrain Laurentia’s position given that the Appalachian margin was at, or equatorward of, the paleolatitude of these terranes. To satisfy these allochthonous data, Laurentia must have moved toward the equator during the Ordovician such that the Appalachian margin was within 10° of the equator by 465 Ma. This movement into the tropics coincided with the collision and exhumation of the Taconic arc system, recorded by a shift in neodymium isotope data from shale on the Appalachian margin to more juvenile values. This inflection in detrital neodymium isotope values precedes a major downturn in global seawater strontium isotopic values by more than one million years, as would be predicted from a change in weathering input and the relatively long residence time of strontium in the ocean. These data are consistent with an increase in global weatherability associated with the tropical weathering of mafic and ultramafic lithologies exhumed during the Taconic arc-continent collision. A Taconic related increase in weatherability is a viable mechanism for lowering atmospheric CO2 levels through silicate weathering contributing to long-term Ordovician cooling. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Zircon U–Pb–Hf isotopic and geochemical characteristics of the Xierzi biotite monzogranite pluton, Linxi, Inner Mongolia and its tectonic implications

The opening, subduction and final closure of the Paleo-Asian Ocean led to the formation of the Central Asian Orogenic Belt. Controversy has long surrounded the timing of final closure of the Paleo-Asian Ocean. Here we present zircon U–Pb ages and petrological, geochemical and in situ Hf isotope data for the Xierzi biotite monzogranite pluton, Linxi, SE Inner Mongolia. U–Pb dating of zircon by LA-ICP-MS yields a middle Permian emplacement age (268.7 ± 2.3 Ma) for the Xierzi pluton that is dominated by biotite monzogranites with high SiO2 (71.2–72.8 wt.%), alkali (Na2O + K2O = 8.05–8.44 wt.%), Al2O3 (14.4–15.2 wt.%) and Fe2O3T relative to low MgO contents, yielding Fe2O3T/MgO ratios of 2.87–3.44, and plotting within the high-K calc-alkaline field on a SiO2 vs. K2O diagram. The aluminum saturation indexes (A/CNK) of the biotite monzogranites range from 1.06 to 1.19, corresponding to weakly to strongly peraluminous. They are enriched in rare earth elements (REE), high field strength elements (HFSEs; Zr, Hf), and large ion lithophile elements (LILEs; Rb, U, Th). The LREEs are enriched relative to the HREEs, with a distinct negative Eu anomaly in a chondrite–normalized REE diagram. Geochemically, the Xierzi biotite monzogranite is classified as an aluminous A-type granite, with all samples plotting within the A2-type granite field on a Y/Nb vs. Rb/Nb diagram. Zircon εHf(t) values and two-stage modal ages of the zircons within the pluton range from +4.80 to +13.65 and from 983 to 418 Ma, respectively, indicating that the primary magma was generated through partial melting of felsic rocks from juvenile crust. Consequently, these results demonstrate that the Xierzi pluton formed under the post-orogenic extensional setting after arc–continent collision in the middle Permian.

Early crustal evolution of the eastern Yangtze Block: Evidence from detrital zircon U-Pb ages and Hf isotopic composition of the Neoproterozoic Huashan Group in the Dahongshan area

The Yangtze Block is separated into the eastern Yangtze Block (EYB) and western Yangtze Block (WYB) by the Chongqing–Huaying line (CHL). An integrated study of detrital zircon Lu-Hf isotopes and U-Pb ages from two sandstones in the Huashan Group together with previous studies reveals six major tectonothermal events in the EYB occurring at about 2.87Ga, 2.67Ga, 2.5Ga, 2.0Ga, 1.84Ga and 0.8Ga. Combined with paleogeographic map, this study reveals distinct detrital zircon U-Pb ages and Hf isotopic characteristics in the western EYB and eastern EYB. The former has most zircon U-Pb ages grouped at about 2.0Ga, 2.67Ga and 2.87Ga with sporadic 2.5Ga ages. The ∼2.5Ga zircons show negative to slightly positive εHf(t) values. However, the latter has significant ages grouped at ∼2.0Ga and ∼2.5Ga with negligible 2.67Ga and older zircon ages. The ∼2.5Ga zircons show negative to high positive εHf(t) values. The Early Neoarchean (2.7–2.6Ga) was a significant period for production of mature continental crust in the western EYB and the majority of crustal materials were extracted from the depleted mantle until then. Zircon Hf isotopic compositions indicate that the Late Neoarchean to Early Paleoproterozoic event (∼2.5Ga) involved both reworking of old crust and juvenile crust growth. With previous studies of sedimentary facies, lithochemical compositions, zircon morphology and zircon Hf isotopes, it is proposed that there are unexposed ∼2.5Ga basement rocks in the EYB. During the 2.2–1.8Ga, subduction processes, collisional and post-collisional events were recorded in the north and west of the EYB, which can be correlated to the assembly and breakup of the supercontinent Columbia. Two changes in εHf(t) value at ∼840Ma and ∼790Ma have been found from detrital zircons in the northern margin of the EYB, which probably represent arc–continent collision and continent–continent collision events, respectively. This long-term subduction process occurring at 1100–790Ma in the EYB appeals for a peripheral position for the Yangtze Block in the supercontinent Rodinia.

Mesozoic radiolarian faunas from the northwest Ilocos Region, Luzon, Philippines and their tectonic significance

AbstractNorthwestern Ilocos Norte in Luzon, Philippines, exposes cherts, peridotite and a variety of metamorphic rocks including chlorite schist, quartzo‐feldspathic schist, muscovite schist and actinolite schist. These rocks are incorporated within a tectonic mélange, the Dos Hermanos Mélange, which is thrust onto the turbidite succession of the Eocene Bangui Formation and capped by the Upper Miocene Pasuquin Limestone. The radiolarian assemblages constrain the stratigraphic range of the cherts to the uppermost Jurassic to Lower Cretaceous. Stratigraphically important species include Eucyrtidiellum pyramis (Aita), Hiscocapsa acuta (Hull), Protunuma japonicus (Matsuoka & Yao), Archeodictyomitra montisserei (Squinabol), Hiscocapsa asseni (Tan), Cryptamphorella conara (Foreman) and Pseudodictyomitra carpatica (Lozyniak). The radiolarian biostratigraphic data provide evidence for the existence of a Mesozoic basinal source from which the cherts and associated rocks were derived. Crucial to determining the origin of these rocks is their distribution and resemblance with known mélange outcrops in Central Philippines. The mélange in the northwestern Ilocos region bears similarities in terms of age and composition with those noted in the western part of the Central Philippines, particularly in the islands of Romblon, Mindoro and Panay. The existence of tectonic mélanges in the Central Philippines has been attributed to the Early to Middle Miocene arc–continent collision. This event involved the Philippine Mobile Belt and the Palawan Microcontinental Block, a terrane that drifted from the southeastern margin of mainland Asia following the opening of the South China Sea. Such arc–continent collision event could also well explain the existence of a tectonic mélange in northwestern Luzon.

Reconstructing in space and time the closure of the middle and western segments of the Bangong–Nujiang Tethyan Ocean in the Tibetan Plateau

When and how the Bangong–Nujiang Tethyan Ocean closed is a highly controversial subject. In this paper, we present a detailed study and review of the Cretaceous ophiolites, ocean islands, and flysch deposits in the middle and western segments of the Bangong–Nujiang suture zone (BNSZ), and the Cretaceous volcanic rocks, late Mesozoic sediments, and unconformities within the BNSZ and surrounding areas. Our aim was to reconstruct the spatial–temporal patterns of the closing of the middle and western segments of the Bangong–Nujiang Tethyan Ocean. Our conclusion is that the closure of the ocean started during the Late Jurassic and was mainly complete by the end of the Early Cretaceous. The closure of the ocean involved both “longitudinal diachronous closure” from north to south and “transverse diachronous closure” from east to west. The spatial–temporal patterns of the closure process can be summarized as follows: the development of the Bangong–Nujiang Tethyan oceanic lithosphere and its subduction started before the Late Jurassic; after the Late Jurassic, the ocean began to close because of the compressional regime surrounding the BNSZ; along the northern margin of the Bangong–Nujiang Tethyan Ocean, collisions involving the arcs, back-arc basins, and marginal basins of a multi-arc basin system first took place during the Late Jurassic–early Early Cretaceous, resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the Southern Qiangtang Terrane on the northern side of the ocean. However, the closure of the Bangong–Nujiang Tethyan Ocean cannot be attributed to these arc–arc and arc–continent collisions, because subduction and the development of the Bangong–Nujiang Tethyan oceanic lithosphere continued until the late Early Cretaceous. The gradual closure of the middle and western segments of Bangong–Nujiang Tethyan Ocean was diachronous from east to west, starting in the east in the middle Early Cretaceous, and being mainly complete by the end of the Early Cretaceous. The BNSZ and its surrounding areas underwent orogenic uplift during the Late Cretaceous.

Plate interior polyphase fault systems and sedimentary basin evolution: A case study of the East Gobi Basin and East Gobi Fault Zone, southeastern Mongolia

Structural interpretation of 2-D seismic reflection data and subsurface-outcrop correlations reveal six distinct phases of deformation recorded in the Paleozoic basement rocks and Mesozoic-Cenozoic basin fill of the East Gobi Basin (EGB), southeastern Mongolia. These phases include arc accretion and arc-continent collision in late Paleozoic time, Late Triassic sinistral shear-zone development, Early Jurassic fold and thrust belt style shortening, Middle Jurassic-Lower Cretaceous extension and rift basin development, middle Cretaceous shortening with basin inversion and regional unconformity development, and Late Cretaceous-Oligocene thermal subsidence with renewed Paleogene left-lateral strike slip faulting across the fault zone. The five post-amalgamation deformation phases are localized along the East Gobi Fault Zone, suggesting that preexisting structures and boundary conditions exert fundamental controls on the long term evolution of intracontinental basins such as the EGB. Subsurface geophysical data and outcrop correlations demonstrate that the main subbasins of the EGB contain major differences in basement metamorphic and structural fabrics, basin fill patterns, and distinct Mesozoic-Cenozoic fault generations. Potential causes related to far-field boundary conditions and other driving factors are suggested for each of the major deformation phases.

Paleoproterozoic metavolcanic rocks in the Ji’an Group and constraints on the formation and evolution of the northern segment of the Jiao-Liao-Ji Belt, China

In this paper we present new petrological and whole-rock geochemical data for the Paleoproterozoic metavolcanic rocks in the lower part of the Ji’an Group within the northern segment of the Jiao-Liao-Ji Belt, China, as well as U–Pb dates and Lu–Hf isotope data for the zircons in these rocks. The new data help in understanding the original nature of these volcanic rocks, as well as their tectonic setting and the formation and evolution of the belt. The zircons can be divided into two groups, of which one is of a typical magmatic origin, whereas the other shows a metamorphic origin. Zircon U–Pb dating and Lu–Hf isotopic results show that the magmatic zircons have peak ages at 2184, 2365, 2503, and 2710Ma with εHf(t) values ranging from +1.40 to +8.95, −10.86 to +1.48, −14.68 to +7.53, and −1.05, and with corresponding model ages of 2.18–2.69, 2.80–3.57, 2.54–3.83, and 3.22Ga, respectively. The metamorphic zircons show two peak ages at 1857 and 1914Ma. Geochemical data show that the protolith of the metavolcanic rocks consists mainly of basaltic andesite, andesite, and dacite, along with minor basalt and rhyolite, and that these rocks formed a continuous calc-alkaline magmatic sequence. In this sequence, the intermediate–basic rocks have relatively low SiO2 contents, are enriched in MgO, FeOT, the light rare-earth elements (LREEs), Cr, Co, and Ni, and depleted in heavy rare-earth elements (HREEs) and high-field strength elements (HFSEs, such as Nb, Ta, and Ti), indicating derivation by partial melting of a depleted lithospheric mantle that had been metasomatized by fluids or melts derived from a subducted slab. On the other hand, the acidic rocks have high contents of SiO2 and low contents of MgO and FeOT, are enriched in alkalis, Ba, Th, U, and K, and depleted in Nb, Ta, P, and Ti. These geochemical characteristics, together with their Hf isotope data, indicate the acidic rocks could have been derived from the partial melting of juvenile crust and a small amount of ancient crustal material, possibly as old as late Archean. We consider, therefore, that the metavolcanic rocks in the lower part of the Ji’an Group were formed at ca. 2.18Ga as an assemblage of volcanic rocks typical of an active continental margin, and that they were affected by two-stage metamorphic events at 1.90 and 1.85Ga. Taking into account the regional geological data, we consider that the evolution of the northern segment of the Jiao-Liao-Ji Belt was related to an arc–continent collision before the middle Paleoproterozoic.

Carboniferous Alaskan-type complex along the Sino–Mongolian boundary, southern margin of the Central Asian Orogenic Belt

We present zircon ages and geochemical data for the Hongshishan Carboniferous Alaskan-type mafic–ultramafic complex exposed in the Beishan area along the Sino–Mongolian boundary, southern margin of the Central Asian Orogenic Belt. This complex mainly consists of dunite, harzburgite, lherzolite, wehrlite, and gabbro, which intrudes Early Carboniferous volcanic rocks and reveals a zoned structure. Zircons of a gabbro sample yielded a 206Pb/238U age of 357 ± 4 Ma, reflecting the time of Early Carboniferous magmatism. Zircon ages were also obtained for an andesite (322 ± 3 Ma) and a basaltic andesite (304 ± 2 Ma). High initial Nd isotope whole-rock values suggest that the Hongshishan gabbro [e Nd(t) = +9.6 − +10.2] and basalt [e Nd(t) = +10.0 − +10.8] were derived from a depleted mantle source. Slightly lower e Nd(t) values for the ultramafic rocks [e Nd(t) = +8.5 − +8.7] suggest some interaction of the parental magma with the continental crust. In contrast, the Late Carboniferous Quershan samples in this area represent subduction-related arc volcanic rocks with Adakite-like compositions. The early Carboniferous Hongshishan Alaskan-type complex was interpreted to represent the remnants of a magma chamber that crystallized at the base of a mature island arc, whereas the Quershan island arc volcanic rocks suggest the resurrection of the subduction process after arc-continent collision and uplift of the roots of the arc.

A reinterpretation of the metamorphic Yuli belt: Evidence for a middle-late Miocene accretionary prism in eastern Taiwan

The Yuli metamorphic belt has been the topic of petrological and geochronological studies for over 40 years and has been interpreted as a Cretaceous melange. Our study utilizes zircon U-Pb dating of schist and exotic blueschist blocks in the Yuli belt. These new ages indicate that these metamorphic rocks are actually middle Miocene in age and may represent the deeper structural levels of an accretionary prism. Several distinctive detrital zircon U-Pb age populations are recognized from 14 siliceous schists of melange-hosted rocks that are similar in age population to the Cretaceous, Eocene-Oligocene, and Miocene strata of Taiwan. The wide range of ages is interpreted as a product mixing of various sedimentary strata prior to metamorphism. Three blueschists of a volcanic-arc protolith enclosed within the host rocks yield crystallization ages of 15.4 ± 0.4, 15.5 ± 0.3, and 16.0 ± 0.2 Ma based on zircon U-Pb dating. In consideration of the new data regarding the Cretaceous-Miocene host rocks and the middle Miocene exotic blueschist blocks, it strongly suggests that the Yuli belt formed at deeper levels of an accretionary wedge during subduction of South China Sea oceanic crust at the middle-late Miocene. Subsequently, the rapid uplift of the metamorphic belt was probably related to doubly vergent wedge extrusion due to the Pliocene arc-continent collision.