Andean-type Subduction Research Articles

Provenance history of the eastern Pearl River Mouth Basin: Implications for the evolution of the South China margin

The provenance history of the Pearl River Mouth Basin is of great significance for understanding how the South China margin evolved from a Mesozoic Andean-type subduction margin into the present-day rifted passive margin. However, the long-term provenance history of continental margin basins in the northern South China Sea is poorly understood. Newly identified Mesozoic−Cenozoic successions from the Lufeng Sag in the eastern Pearl River Mouth Basin provide valuable records of the provenance history and related evolution of the South China margin. Here, we trace the provenance of the eastern Pearl River Mouth Basin using U-Pb ages of zircon and ZTR values (proportions of zircon, tourmaline, and rutile in all transparent heavy minerals) of heavy minerals, and determine the tectonic setting and crustal thickness using trace element ratios and Eu anomalies in zircon. The results show that from the middle Eocene strata—which have a U-Pb age spectrum similar to strata of the Upper Cretaceous—to the Miocene strata, the proportion of Mesozoic age populations gradually increases, while the proportion of Paleozoic age populations decreases. In addition, following the middle Eocene high values, the ZTR values decreased during the late Eocene and then increased during the early Oligocene. Based on the variations in zircon age populations and ZTR values, the middle Eocene sediment is interpreted to have been recycled from Upper Cretaceous to middle Eocene successions, followed by an early Oligocene provenance change from intrabasinal to extrabasinal sources. Moreover, the zircon trace element ratios and Eu anomalies indicate that a widespread Mesozoic continental magmatic arc formed before ca. 90 Ma along the South China margin, and the crust simultaneously thickened during this time. In accordance with the provenance history, the evolution of the South China margin is divided into three tectonic phases: the Paleo-Pacific, Proto-South China Sea, and South China Sea. During the transition from the Paleo-Pacific to the Proto-South China Sea tectonic phase, sediment was recycled from Upper Cretaceous to middle Eocene successions in the eastern Pearl River Mouth Basin. During the transition from the Proto-South China Sea to the South China Sea tectonic phases, the provenance changed from intrabasinal to extrabasinal sources.

Plate tectonic limits on the assembly of cratonic Australia

Continuous Proterozoic plate reconstructions consistent with plate geodynamics were designed to reconcile data, conceptual models, and human imagination as applied to the assembly of the Australian craton. The reconstructions were created using GPlates and considered available magmatic and metamorphic U-Pb zircon data. Two tectonic scenarios for the Paleo- to Mesoproterozoic assembly of the cratonic lithosphere of Australia were tested for their geodynamic and geochronological viability. The scenarios are founded on tectonic events that are relatively well-established and well-supported by data, but also with several periods and areas that are disputed mostly due to scarcity of data. For these scenarios, we test geodynamic criteria to explore a) which possibilities are better supported by our geodynamic criteria, and b) the implications of those models for the larger-scale plate tectonic setting. The time frame in this model set includes assembling two supercontinents, Nuna (1900–1800 Ma) and Rodinia (1300–900 Ma), and the transition between them. Model 1 presents a progressive assembly of the Australian continent between 2000 Ma and 1300 Ma, showing events in the West Australian Craton (WAC), the convergence of the WAC and the North Australian Craton (NAC), and then accretion at the margin of the South Australian Craton and NAC until collision in the Albany-Fraser Orogen (AFO). Model 2 suggests WAC was separate from the rest of the Australian Craton until 1300 Ma and all combined in one orogenic cycle. It proposes Andean-type subduction along the E and NE margins of the WAC for 40–60 million years as the means of final assembly. Both models are geodynamically plausible, yet each lacks sufficient data support to conclusively resolve the differences and deepen our understanding of the Proterozoic assembly of Australia.

The Cambrian Atlas – Ossa–Morena – North Armorican Rift, West Gondwana: along- and off-axis stratigraphic and volcano-tectonic patterns

Abstract The Cambrian Atlas – Ossa–Morena – North Armorican Rift extended along West Gondwana from the end of the Pan-African and Cadomian orogenies until the diachronous beginning of drift conditions related to the opening of the Rheic Ocean. The along-axis rift cross-cut the western parts of the Anti-Atlas, High Atlas and Coastal Meseta, which were linked to the Ossa–Morena Zone and the North Armorican Domain, whereas several joint tectonic branches connected with off-axis rift transects of the Central Iberian, West Asturian–Leonese and Cantabrian zones (Iberian Massif), the Central and South Armorican domains, the Occitan Domain, the Pyrenees, and southern Sardinia. The pre-rift unconformity, post-dating the orogenic collapse, is characterized by initial (half-)graben development and subsequent infill, with slope-related breccias and conglomerates controlled by the denudation of surrounding uplands. Synrift pulses show regional extension and are distinctly identifiable on the top of rift shoulders, recording episodes of carbonate production due to their association with karst and hydrothermal processes. The break-up unconformity ranges from volcanic-free angular discordances and paraconformities to generalized uplift and denudation of subaerially exposed areas, associated with the onset of granite-dominant large igneous provinces (LIPs). The Furongian–Tremadocian (Toledanian) and Ordovician (Sardic) phases have been interpreted as due to: (i) Andean-type subduction magmatism reaching the crust in an arc–back-arc setting; (ii) post-collisional decompression melting without significant mantle involvement; and (iii) partial melting of the lower continental crust affected by the underplating of hot mafic magmas linked to superplumes.

Late Paleoproterozoic ultrahigh-temperature mafic granulite from the eastern Alxa Block, North China Craton

The timing and mechanism of ultrahigh-temperature (UHT) granulite facies metamorphism is critical to understanding the tectono-thermal evolution of granulite terranes and their relationships to supercontinent assembly. Here we document late Paleoproterozoic UHT mafic granulites from the Diebusige Complex in the eastern Alxa Block, North China Craton. These rocks occur as lens within felsic gneiss, marble, and garnet-bearing pyroxenite and contain representative granulite-facies minerals such as garnet, clinopyroxene, plagioclase, and Ti-rich amphibole. Based on phase equilibria modelling, Ti-in-amphibole (Amp) thermometer, and rare earth element (REE)-based thermometers, UHT metamorphic condition of 948 – 1048 ℃/11 – 14 kbar was obtained for the peak metamorphic stage, which was followed by retrograde metamorphism under lower amphibolite – greenschist facies (401 – 565 ℃/5.2 – 7.1 kbar). Zircon U-Pb dating and trace element systematics suggest that the protolith of the studied UHT mafic granulites emplaced at ∼ 1.97 Ga. This is followed by rapid metamorphism to UHT conditions and slow cooling in the lower crust during 1.96 – 1.86 Ga, suggesting a long-lived hot orogen in the eastern Alxa Block, possibly resulting from Andean-type subduction – accretion in the eastern Alxa Block. Combining chronological and metamorphic P-T data on UHT metamorphism from adjacent areas, our results support a long-lived late Paleoproterozoic (ca. 2.0 – 1.8 Ga) hot orogen that extends at least from the Khondalite Belt to the eastern Alxa Block.

Neoproterozoic reorganization of the Circum- Mozambique orogens and growth of megacontinent Gondwana

The serpentine orogenic belts that formed during the Neoproterozoic assembly of Gondwana resulted in geodynamic changes on the planet in advance of the Cambrian radiation. The details of Gondwana assembly associated with the closure of the Mozambique Ocean are enigmatic. We compile published geological and paleomagnetic data to argue that the Tarim block was associated with the Azania and Afif–Abas–Lhasa terranes and they were the locus of long-lived Andean-type subduction during the ~900–650 Ma interval. Our model suggests a subduction system reorganization between 750-720 Ma, which resulted in two distinct phases of Mozambique ocean evolution. Between 870-750 Ma, a N-S oriented subduction system marks the locus of ocean crust consumption driven by the extension of the Mozambique Ocean. Beginning ~720 Ma, a newly developed ~E-W oriented subduction system began to consume the Mozambique Ocean and led to the assembly of eastern Gondwana. Our new reconstruction uses true polar wander to constrain the relative paleolongitude of Tarim, South China and West Africa. In this scenario, the closure of the Mozambique Ocean and formation of Gondwana was orthogonal to the preceding supercontinent Rodinia.

Molybdenum cycling in Andean-type subduction and metallogenic implications

Molybdenum cycling in Andean-type subduction and metallogenic implications

Differential fold interference patterns in an intracontinental orogen: Insights from superposed buckle folding in southern Jiangxi Province, South China Block

The South China Block (SCB) experienced multi-stage tectono-magmatic events during the Mesozoic, forming a broad and episodic intracontinental orogenic belt. It is controversial whether the driving force of the Mesozoic intracontinental orogeny in the SCB is related to the far-field effects of plate convergence. To better understand the driving mechanism of intracontinental orogeny, we conducted a detailed structural investigation of the Xingguo area in southern Jiangxi Province, located in the central part of the SCB. Two regional-scale buckling superposed folds were identified as the Chayuan arcuate syncline of the fold axis protruding to the north (Type 2a interference pattern) and the Xiefang syncline of the fold axis extending NW–SE (Type 1d interference pattern). Although there are differences in fold interference patterns, the Chayuan arcuate syncline and Xiefang syncline were formed by the superimposition of the Middle–Late Triassic NE–SW shortening and the Middle–Late Jurassic nearly E–W shortening. This phenomenon of the differential fold interference patterns in the same tectonic setting is determined by the difference in geometric characteristics of their initial folds. Combined with the variation of the Mesozoic paleostress field, it is considered that the Mesozoic intracontinental orogeny in the SCB is mainly controlled by the far-field stress propagation generated by plate interactions. Based on the analysis of tectonic architecture, we propose that the Mesozoic tectonic evolution of the SCB experienced a transformation from multi-plate convergence in the Triassic to Andean-type subduction in the Jurassic. This tectonic transformation finally resulted in the reactivation of the Precambrian multi-terrane collage of the SCB.

Highly evolved miaskitic syenites deciphering the origin and nature of enriched mantle source of ultrapotassic magmatism in the Variscan orogenic root (Bohemian Massif, Moldanubian Zone)

A detailed study of alkali-rich syenites from the Gföhl unit of the Moldanubian Zone in the Bohemian Massif provides constraints on the exact origin and character of mantle source of ultrapotassic magmatism in the Variscan orogenic root in Central Europe. The syenites are characterized by highly alkaline composition (K2O 11.3–12.7 wt%; K2O/Na2O 6.8–7.5), crust-like isotopic signatures (87Sr/86Sr335 ∼ 0.7116; εNd335 ≤ −7.8), and significant trace element enrichment (Th, U, Zr, Hf, LILE, LREE). They host an extraordinary amount of zircon (0.5–5 vol%) along with subordinate titanite and apatite, reflecting their miaskitic affinity. Mineral chemistry suggests that syenites formed through fractional crystallization and accumulation from highly reduced, metaluminous to slightly peraluminous mantle-derived alkaline magmas. Whole-rock geochemistry and Sr-Nd isotopes provide direct evidence that they were produced by partial melting of metasomatic phlogopite-bearing vein network (glimmerites) in the lithospheric mantle, generated by the interaction of (U)HP fluids/melts derived from deeply subducted crustal material (Moldanubian granulites) with wall-rock peridotites. Low-degree partial melting of pure vein component produced unusual geochemical signatures of miaskitic syenites, compared to the composition of common ultrapotassic rocks in the Bohemian Massif, reflecting relatively higher degrees of partial melting of mixed glimmerite-peridotite mantle source.The emplacement of alkali-rich syenites was almost contemporaneous with the mantle source enrichment and closely followed by initial fast cooling dated by 40Ar/39Ar amphibole-biotite ages ranging between 329.8 ± 1.6 and 331.4 ± 0.7 Ma, corresponding to rapid exhumation of the Variscan orogenic root in Central Europe. The U-Pb apatite age of 305.9 ± 5.3 Ma likely reflects further cooling to lower temperatures. The syenite emplacement was linked to the early impulse of ultrapotassic magmatism associated with the Andean-type subduction of the Saxothuringian domain beneath the Moldanubian block. The close temporal association of K-rich magmatism in the Saxothuringian and Moldanubian Zone is indicated by identical cooling ages of miaskitic syenites and other Saxonian ultrapotassic rocks, as revealed by 40Ar/39Ar dates around 330 Ma.

Short-lived intra-oceanic arc-trench system in the North Qaidam belt (NW China) reveals complex evolution of the Proto-Tethyan Ocean

Abstract Recognition of any intra-oceanic arc-trench system (IOAS) could provide invaluable information on the tectonic framework and geodynamic evolution of the vanished ocean basin. The Tanjianshan Complex and mafic-ultramafic rocks along the North Qaidam ultra-high pressure metamorphic belt in NW China record the subduction process of the Proto-Tethyan Ocean. Four lithotectonic units, including island arc, ophiolite, forearc basin, and accretionary complex, are recognized based on detailed field investigation. They rest on the northern margin of the Qaidam block and occur as allochthons in fault contact with underlying high-grade metamorphic rocks. The ophiolite unit mainly consists of ultramafic rocks, 527–506 Ma gabbro, 515–506 Ma plagiogranite, dolerite, and massive lava. High-Cr spinels in serpentinite, dolerite with forearc basalt affinity, and boninitic lava collectively indicate a forearc setting. The accretionary complex, exposed to the south of the ophiolite complex and island arc, is highly disrupted and contains repeated slices of basalt, 495–486 Ma tuff, chert, limestone, and mélange. Tuffs with positive zircon εHf(t) values indicate derivation from a nearby juvenile island arc. These lithotectonic units, as well as the back-arc basin, are interpreted to constitute a Cambrian IOAS that formed during the northward subduction of the Proto-Tethyan Ocean. Combined with regional geology, we propose a new geodynamic model involving short-lived Mariana-type subduction and prolonged Andean-type subduction to account for the complex evolution of the Proto-Tethyan Ocean. The reconstruction of a relatively complete IOAS from the North Qaidam belt not only reveals a systematic evolution of intra-oceanic subduction but also advances our understanding of the subduction and accretion history of the Proto-Tethyan Ocean.

Late Triassic rift tectonics at the northernmost Andean margin (Sierra Nevada de Santa Marta)

Break-up of western Pangaea initiated a back-arc type tectonic setting along the northwestern margin of Gondwana, which resulted in the accumulation of sedimentary and volcanic deposits in extensional basins during the Middle-Late Triassic. The Sierra Nevada de Santa Marta massif, at the northwestern termination of the Andes, exposes Upper Triassic syn-rift successions beneath a voluminous Lower Jurassic volcanic cover. In this contribution, we present a sedimentological and stratigraphical analysis of the Upper Triassic Los Indios and Corual formations, by integrating whole-rock geochemistry, zircon U–Pb geochronology, and a biostratigraphic revaluation of the conchostracan fossil content. We divide the Los Indios Formation into two sandy to conglomeratic and two muddy members. First and third members were deposited in a coarse-grained delta environment. The second and fourth members are interpreted to represent offshore deposits resulting from the encroachment of gravity-flow lobes. Volcanoclastic and volcanic rocks interbedded with sedimentary deposits of the Los Indios and Corual formations record bimodal basic-acid volcanism with a variably alkaline tendency. Based on their alkaline and calc-alkaline signature, and rare earth and trace element patterns, volcanic rocks display a back-arc affinity. With respect to the age of the Los Indios Formation, we recovered conchostracans diagnostic of Upper Carnian and Lower Norian fauna, bearing similarity with faunal assemblages of other Upper Triassic deposits exposed in the northern Andes. The Los Indios and Corual formations are considered as syn-rift successions in a back-arc extension setting. The development of this back-arc basin coincided with the Late Triassic separation of the Oaxaquia, Acatlán, and Chortis terranes during Pangaea break-up. Finally, the Early to Middle Jurassic evolution of the NW Gondwana margin is marked by the inception of a magmatic arc, which characterizes the establishment of Andean-type subduction coeval to ignimbrite flare-up.

Cretaceous exhumation of the Triassic intracontinental Xuefengshan Belt: Delayed unroofing of an orogenic plateau across the South China Block?

A large plateau can be produced by crustal thickening in convergent zones such as continental collision belts and Andean-type subduction zones, but the life cycles of such plateaux are not well-understood. In particular, it is not clear how long they persist after construction, before other tectonic processes or erosion reduce crustal thickness and elevation to near-normal levels. Triassic subduction- and collision-tectonics produced intense deformation, magmatism and metamorphism across the entire South China Block. This large-scale crustal shortening created a broad orogenic belt, uplifted most parts of the South China Block, and probably initiated the growth of an orogenic plateau. Our study presents low-temperature thermochronology data from the Xuefengshan Belt in the interior of the South China Block. There was along-strike variation in exhumation. The north orogenic core was subjected to Triassic (~245–210 Ma), and Late Cretaceous (~100–80 Ma) exhumation, whereas the cooling path of the south orogenic core reflects a two stage Cretaceous evolution. The variable exhumation pattern reflects non-uniform tectonics in different regions, but both regions were subject to Late Cretaceous extension. We tentatively reconstruct the original plateau paleo-elevation to be ~1.5 km above sea level, based on the amount of exhumation (~10 km) and the present crustal thickness (~35 km). The T-t trajectories of the Xuefengshan Belt and other Triassic belts highlight the significance of Cretaceous extension and exhumation in shaping the tectonic configuration of the South China Block. Large-scale extension was probably triggered by rollback of the Paleo-Pacific subduction zone.

Gold endowments of porphyry deposits controlled by precipitation efficiency

Porphyry deposits are natural suppliers of most copper and significant gold to our society. Whereas the Cu-richest (Au-poor) porphyries are related to Andean-type subduction and typical calc-alkaline magmatism, the Au-richest porphyries are associated with high-K calc-alkaline to alkaline magmatism in late to post-subduction or post-collision and extensional settings, and subordinately with calc-alkaline magmatism. The reasons behind these associations and the large variations in metal endowments of porphyry Cu–Au deposits remain obscure. Here, I show that porphyry Cu–Au deposits define two distinct trends in Au vs. Cu tonnage plots (Cu-rich and Au-rich). Metal endowments for both trends grow larger the longer the mineralization process. However, Au is precipitated at much higher rates in Au-rich than in Cu-rich porphyry deposits. Using Monte Carlo simulations of petrologic processes, I show that whereas Cu-rich porphyries require large amounts of magma and water to be formed, Au-rich porphyries are the result of a better efficiency of Au precipitation.

Geometry and development of a hybrid thrust belt in an inner forearc setting: Insights from the Potrerillos Belt in the Central Andes, northern Chile

Thick- and thin-skinned “hybrid” thrust belts are typically developed in areas where crustal shortening was preceded by extension and normal faulting. They have been recognized in many places within the Central Andean backarc regions of Perú, Argentina, and Bolivia. In contrast, these structures have not been broadly studied in forearc regions. To understand the geometry and kinematics of this style of tectonism in a forearc setting related to Andean-type subduction zones, we used field and industrial 2D seismic data to study the Potrerillos thrust and fold belt and the Salar de Pedernales Basin in northern Chile (26°S). Our results indicate that the structure in this region consists of a dominantly east-verging contracting system composed of large basement thrusts, Mesozoic inverted normal faults, and shallow thrust-related folds. Large basement thrust ramps and imbricated wedges represent the most prominent basement structures, which accommodated major crustal shortening. Reactivated and tectonically inverted Mesozoic extensional structures also constitute an important structural component, especially beneath the Salar de Pedernales Basin. These exhibit asymmetrical inverted anticlines with arrowhead shapes on the hanging walls of the inverted normal faults. The subsidiary shallow thrust-related folds commonly display fault bend and propagation folds that primarily accommodated the shortening of the Mesozoic and Cenozoic stratified deposits overlying the basement thrusts and cored anticlines. The age of the contraction in this region is poorly constrained; however, many of the oldest sequences accumulated in the top of the syn-rift sequences and related to synorogenic deposits, can be correlated with those identified in neighboring regions (e.g., Salar de Atacama, Salar de Punta Negra basins and Frontal Cordillera, among others), which have reported Upper Cretaceous ages. Based on this stratigraphic correlation we suggest that orogenesis may have begun in the Late Cretaceous with basin inversion, and then continued during the Cenozoic with basement thrusting.

The tectonic controls on the Paleoproterozoic volcanism and the associated metallogeny in the South Amazonian craton, Brazil: Sr–Nd–Pb isotope constraints

Whole-rock Sr–Nd–Pb isotope data for the Sobreiro and Santa Rosa volcanic sequences and Archean basement rocks near São Félix do Xingu city, northern Brazil, provide constraints for the colossal Proterozoic effusive and explosive well-preserved volcano–plutonic magmatism that covers about 1,500,000 km2 of the Amazonian craton. The basal, 1.88 Ga high-K calc-alkaline andesitic–dacitic Sobreiro Formation shows narrow variations in isotopic ratios [(87Sr/86Sr)i = 0.70352 to 0.70518; (143Nd/144Nd)i = 0.509692 to 0.509972; (206Pb/204Pb)i = 16.0492 to 16.3041; and (207Pb/204Pb)i = 15.6265 to 15.7505] that suggest mixing between the DMM-B and EM-1 and/or EM-2 domains in the melting source. The integration with published data reveals geochemical affinity of the Sobreiro Formation with Andean-type subduction; episodes of subduction angle flattening; and forearc subduction erosion. The latter caused the input of crustal-derived material and oceanic sediments in the trench and mixing with mantle-sourced magmas, a common mechanism active in modern orogenic environments. The upper, fissure-controlled, 1.87 Ga A-type rhyolitic Santa Rosa Formation reveals a high initial ratio [(87Sr/86Sr)i = 0.70949 and a large range of values for (143Nd/144Nd)i = 0.509624 to 0.509909; (206Pb/204Pb)i = 16.7271 to 21.7086; and (207Pb/204Pb)i = 15.8421 to 17.2780], compatible with partial melting of several Archean crustal sources and terrigenous sediments. We supply interpretations relative to tectonism, magma origin and evolution, and formation of arc-related world-class precious and base metals deposits, analogous to the events that occurred in the Andean belt and western North America. It represents new support for more systematic mineral exploration with modern approaches of these poorly studied, but nonetheless promising, Amazonian Proterozoic units.

Mesozoic tectonic evolution of the Proto-South China Sea: A perspective from radiolarian paleobiogeography

The Proto-South China Sea (PSCS) represents the lithosphere that formerly occupied the region southeast of South China where the present-day South China Sea is located. The subduction and demise of the PSCS have attracted wide attention, whereas the pre-subduction history (particular the Mesozoic) of the region remains relatively neglected. In this paper, the radiolarian paleobiogeography of the region is analyzed to identify the paleobiogeographic affinity and paleo-connectivity of the PSCS with the Pacific and Tethys oceans during the Mesozoic. Then the continent–ocean configuration and tectonic evolution of the PSCS and adjacent areas are discussed through the integration of paleobiogeographical results and associated paleomagnetic, stratigraphic, and petrologic data. The main conclusions are as follows: (1) During the Triassic, the PSCS was situated in the transition zone between the Pacific and Tethys oceans, and its paleobiogeography was influenced by both oceans. The PSCS was characterized by complex tectonic overprinting by the Pacific and Tethyan tectonic regimes. (2) In the Early–Middle Jurassic, the PSCS was well connected to the Pacific Ocean but poorly connected to the Tethys Ocean, consistent with the paleobiogeography of the PSCS being mainly constrained by the Pacific Ocean. The PSCS was part of the Paleo-Pacific slab. (3) During the Late Jurassic–Cretaceous, the PSCS was in a semi-closed environment. The Late Jurassic–early Late Cretaceous PSCS was part of a west-facing Andean-type subduction zone, and the late Late Cretaceous PSCS was situated in an extensional tectonic setting.

Paleogene evolution of the Burmese forearc basin and implications for the history of India-Asia convergence

The geological history of the Burmese subduction margin, where India obliquely subducts below Indochina, remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader Tibetan-Himalayan orogen. Various scenarios for the evolution of the orogen have been proposed, including a collision of India with Myanmar in the Paleogene, a significant extrusion of Myanmar and Indochina from the India-Asia collision zone, or very little change in paleogeography and subduction regime since the India-Asia collision. This article examines the history of the Burmese forearc basin, with a particular focus on Eocene – Oligocene times to reconstruct the evolution of the Burmese margin during the early stages of the India-Asia collision. We report on sedimentological, geochemical, petrographical, and geochronological data from the Chindwin Basin - the northern part of the Burmese forearc - and integrate these results with previous data from other basins in central Myanmar. Our results show that the Burmese margin acted as a regular Andean-type subduction margin until the late Middle Eocene, with a forearc basin open to the trench and fed by the denudation of the Andean volcanic arc to the east. We show that the modern tectonic configuration of central Myanmar was formed 39-37 million years ago, when the Burmese margin shifted from an Andean-type margin to a hyper-oblique margin. The forearc basin was quickly partitioned into individual pull-apart basins, bounded to the west by a quickly emerged accretionary prism, and to the east by synchronously exhumed basement rocks, including coeval high grade metamorphics. We interpret this shift as resulting from the onset of strike-slip deformation on the subduction margin leading to the formation of a paleo-sliver plate, with a paleo fault system in the accretionary prism, pull apart basins in the forearc, and another paleo fault system in the backarc. This evolution implies that hyper-oblique convergence below the Burmese margin is at least twice older than previously thought. Our results reject any India-Asia convergence scenario involving an early Paleogene collision of India with Myanmar. In contrast, our results validate conservative geodynamic models arguing for a close-to-modern precollisional paleogeography for the Indochina Peninsula, and indicate that any postcollisional rotation of Indochina, if it occurred at all, must have been achieved by the late Middle Eocene.

Ediacaran magmatism in the North Lhasa terrane, Tibet and its tectonic implications

The origin of the North Lhasa terrane in Tibet remains enigmatic. Our study reports the first evidence for Ediacaran magmatic rocks, including gabbros (ca. 572 Ma), tonalites (ca. 568 Ma), and rhyolites (ca. 547–541 Ma), in the North Lhasa terrane, Tibet. Gabbros are calc–alkaline and exhibit arc-like (e.g., enrichments of Rb and Pb and depletions of Nb, Ta, P, and Ti) geochemical affinities. These gabbros have variable zircon εHf(t) values (−5.5 to +0.9) and they are probably derived from partial melting of an enriched lithospheric mantle source triggered by a subduction event. Tonalites are high-K calc–alkaline and exhibit negative zircon εHf(t) values (−9.1 to −0.8). They can be attributed to partial melting of a thickened lower continental crust with minor mantle-derived component. Rhyolites have negative zircon εHf(t) values (−6.1 to −1.2) and they were probably products of partial melting of Paleoproterozoic metasedimentary rocks. These Ediacaran magmatic rocks were interpreted as a result of the Andean-type subduction along the Gondwanan Proto-Tethyan margin. Coeval Andean-type magmatism has been documented along the northern Arabian margin but absent in the northern India or Australia. Integrating previous studies with the data presented in this contribution, we suggest that the North Lhasa terrane was most likely located in the transitional area between the Arabian and Indian–Australian proto-Tethyan margins in the Ediacaran and it originated from the northern segment of the East African orogen.

Evolution of Eocene to Oligocene arc-related volcanism in the North Patagonian Andes (39–41°S), prior to the break-up of the Farallon plate

Voluminous Paleogene magmatic rocks (44 to 29Ma) are found in a retroarc position in the Northern Patagonian to Southern Central Andes (~39–42°S), whose origin remains controversial. Geochemical data in these Eocene to Oligocene volcanic associations are herein used to unravel their origin and understand changes in subduction parameters. Geochemical signatures indicate arc-related associations and reflect changing geodynamic boundary conditions of the Andean margin through time. In particular, Eocene magmatism (~44Ma; Pilcaniyeu Belt) shows an alkaline-like signature and limited slab influence. Reported contemporaneous within-plate magmatism (~47–43Ma) in an easternmost position reflects a more typical enriched source. Oligocene arc-like volcanism (~29Ma; El Maitén Belt), which developed in an extensional retroarc setting, shows a higher contribution from slab-derived fluids and a calc-alkaline source. A comparison with younger arc-related magmas from the region (~26–20Ma), emplaced in an intra- to retroarc position (Cura Mallín and Abanico basins), indicates a progressive increase in slab-signature, associated with a tholeiitic magma source. We propose that these compositional variations could be directly related to changes in plate configuration before and after the Farallon plate break-up and the initiation of a more orthogonal convergence typical of the present Andean-type subduction zone.

Geochemical constraints on the origin of Late Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China

An integrated study of zircon U–Pb ages, Hf–O isotopes, whole-rock major-trace elements and Sr–Nd–Hf isotopes was carried out for Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China. The results not only provide insights into the origin of anorogenic magmatism in the Middle–Lower Yangtze Valley but also shed light on the petrogenesis of continental arc andesites in general. The Ningwu andesites are mainly composed of trachyandesite and trachyte, with subordinate of basaltic trachy-andesites. They exhibit variable and low MgO contents of 0.1–3.7 wt.% and low Mg# values of 1.6–51.2, high K2O contents of 2.0–9.9 wt.%. They are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Furthermore, these rocks exhibit relatively enriched whole-rock Sr–Nd–Hf–Pb isotope compositions, with initial 87Sr/86Sr ratios of 0.7050–0.7082, negative εNd(t) values of −8.1 to −1.8, and negative εHf(t) values of −6.7 to 0.4. Zircon U–Pb dating yields consistent ages of 128±2 to 133±2Ma for magma emplacement through volcanic eruption. The zircon exhibits slightly high δ18O values of 5.7–7.8‰ and variable εHf(t) values of −5.8 to 0.7. All andesites can be categorized into two groups in view of their distinct geochemical compositions, an integrated interpretation of which allows deciphering of their petrogenesis. Group I andesite was produced by a relatively larger extent of partial melting of a more fertile and enriched mantle source that contains a greater amount of phlogopite, whereas Group II andesite was generated by a relatively smaller extent of partial melting of a less fertile and enriched mantle source that contains a less amount of phlogopite. In either case, these two mantle sources are less ultramafic in lithochemistry than normal mantle peridotite due to its reaction with metasediment-derived hydrous felsic melts. This metasomatism would take place during subduction of the Cathaysia oceanic crust beneath the Yangtze Craton in the Neoproterozoic, whereas the Mesozoic mafic magmatism in the Middle–Lower Yangtze Valley is essentially caused by reactivation of the fertile and enriched mantle sources due to lithospheric extension in response to the subduction of Pacific plate beneath the eastern China continent. Therefore, the geochemical composition of andesites is primarily governed by their mantle sources that were generated by reaction of the mantle wedge peridotite with hydrous felsic melts originated from the subducted terrigenous sediment in Andean-type subduction zones, though such sources may be partially melt either immediately following generation or at a later time. Late-stage magma evolution processes such as fractional crystallization and magma mixing would have further modified the lithochemical composition of andesites but they are not the governing factors for their trace element and radiogenic isotope compositions.

U-Pb age and Hf-O isotopes of detrital zircons from Hainan Island: Implications for Mesozoic subduction models

A compilation of magmatic ages from the Mesozoic South China Block suggests a number of “magmatic quiescence” periods at ca. 205–195Ma, ca. 150–140Ma and ca. 125–115Ma, casting doubt on tectonic models that suggest ongoing Andean-type subduction along the South China continental margin. However, SIMS U-Pb analyses on two detrital zircon samples from the Cretaceous Lumuwan Formation on Hainan Island, southeast China, reveal three major age peaks at ca. 120Ma, ca. 155Ma and ca. 235Ma. Zircons of these ages are mostly euhedral and show typical magmatic oscillatory zoning, suggesting short-distance transport from nearby magmatic sources. The extremely rare occurrence of ca. 120Ma magmatic records onshore suggests that detrital zircons of this age population may be derived from a source proximal to Hainan Island but presently missing. Therefore, our data provide new evidence for ongoing magmatic activity in late Mesozoic South China.In situ Hf and O isotope analyses of the Mesozoic detrital zircons reveal large variations in both εHf(t) (−21.2 to 10.5) and δ18O (4.4‰ to 13.6‰) values. A general negative correlation between them suggests the reworking of old supracrustal materials (average crustal residence age of ca. 2.0Ga) by juvenile mantle-derived magmas. The progression of increasing εHf(t) and decreasing δ18O values of zircons from the Triassic to the Cretaceous suggests progressive crustal growth during the Mesozoic. The results are consistent with hybridization at an active continental margin. We briefly review tectonic models for the Indosinian orogeny and suggest that the petrologic evidence indicates that Mesozoic magmatism was part of the circum-Pacific accretionary orogens that formed along the continental margin of East Asia no later than ca. 250Ma and continued at least to the late Cretaceous.