Indosinian Orogeny Research Articles

Provenance analysis of the Late Triassic Yichuan Basin: constraints from zircon U-Pb geochronology

AbstractLaser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating has been performed on detrital zircons from the Chunshuyao Formation sandstone of Yichuan Basin. The ages of 85 detrital zircon grains are divided into three groups: 252-290 Ma, 1740-2000 Ma, and 2400-2600 Ma. The lack of Early Paleozoic and Neoproterozoic U-Pb ages indicates that there is no input from the Qinling Orogen, because the Qinling Orogen is characterized by Paleozoic and Neoproterozoic material. In combination with previous research, we suggest that the source of the Chunshuyao Formation is most likely recycled from previous sedimentary rocks from the North China Craton. In the Late Triassic, the Funiu ancient land was uplifted which prevented source material from the Qinling Orogen. Owing to the Indosinian orogeny, the strata to the east of the North China Craton were uplifted and eroded. The Yichuan Basin received detrital material from the North China Craton.

The Indosinian orogeny: A perspective from sedimentary archives of north Vietnam

The Triassic stratigraphic framework for the Song Da and the Sam Nua basins, north Vietnam, suffers important discrepancies regarding both the depositional environments and ages of the main formations they contain. Using sedimentological analyses and dating (foraminifer biostratigraphy and U-Pb dating on detrital zircon), we provide an improved stratigraphic framework for both basins. A striking feature in the Song Da Basin, located on the southern margin of the South China Block, is the diachronous deposition, over a basal unconformity, of terrestrial and marine deposits. The sedimentary succession of the Song Da Basin points to a foreland setting during the late Early to the Middle Triassic, which contrasts with the commonly interpreted rift setting. On the northern margin of the Indochina Block, the Sam Nua basin recorded the activity of a proximal magmatic arc during the late Permian up to the Anisian. This arc resulted from the subduction of a southward dipping oceanic slab that separated the South China block from the Indochina block. During the Middle to the Late Triassic, the Song Da and Sam Nua basins underwent erosion that led to the formation of a major unconformity, resulting from the erosion of the Middle Triassic Indosinian mountain belt, built after an ongoing continental collision between the South China and the Indochina blocks. Later, during the Late Triassic, as syn- to post-orogenic foreland basins in a terrestrial setting, the Song Da and Sam Nua basins experienced the deposition of very coarse detrital material representing products of the mountain belt erosion.

Petrologic and zircon U–Pb geochronological characteristics of the pelitic granulites from the Badu Complex of the Cathaysia Block, South China

The recognition of the Indosinian Orogeny in the South China block has been controversial and difficult because of strong weathering and thick cover. High temperature (HT) and high pressure (HP) metamorphic rocks related to this orogeny were considered to be absent from this orogenic belt until the recent discovery of eclogite and granulite facies meta–igneous rocks, occurring as lenses within the meta–sedimentary rocks of the Badu Complex. However, metamorphic state of these meta–sedimentary rocks is still not clear. Besides, there have been no geochronological data of HT pelitic granulites previously reported from the Badu Complex. This paper presents petrographic characteristics and zircon geochronological results on the newly discovered kyanite garnet gneiss, pyroxene garnet gneiss and the HT pelitic granulites (sillimanite garnet gneiss). Mineral assemblages are garnet + sillimanite + ternary feldspar + plagioclase + quartz + biotite for the HT pelitic granulite, kyanite + ternary feldspar + garnet + sillimanite + plagioclase + quartz + biotite for the kyanite garnet gneiss, and garnet + biotite + pyroxene + plagioclase + ternary feldspar + quartz for the pyroxene garnet gneiss, respectively. Decompressional coronas around garnet grains can be observed in all these pelitic rocks. Typical granulite facies mineral assemblages and reaction textures suggest that these rocks experienced HP granulite facies metamorphism and overprinted decompression along a clockwise P–T loop. Results from integrated U–Pb dating and REE analysis indicate the growth of metamorphic zircons from depleted heavy REE sources (100–50 chondrite) compared with detrital zircons derived from granitic sources (typically > 1000 chondrite). Metamorphic zircons in HP granulite exhibit no or subdued negative Eu anomalies, which perhaps indicate zircon overgrowth under eclogite facies conditions. The zircon overgrowth ages range from 250 to 235 Ma, suggesting that HP granulite (eclogite) to granulite facies metamorphism of these supracrustal rocks occurred in the Early–Middle Triassic. Based on the presence of HP granulite facies pelitic rocks, it is inferred that significant underthrusting was involved during the Indosinian Orogeny which introduced these supracrustal rocks to lower crustal levels.

Geodynamics of the Indosinian orogeny between the South China and Indochina blocks: Insights from latest Permian–Triassic granitoids and numerical modeling

Geodynamics of the Indosinian orogeny between the South China and Indochina blocks: Insights from latest Permian–Triassic granitoids and numerical modeling

The genesis of granite-related hydrothermal uranium deposits in the Xiazhuang and Zhuguang ore fields, North Guangdong Province, SE China: Insights from mineralogical, trace elements and U-Pb isotopes signatures of the U mineralisation

Granite-related hydrothermal U deposits from the Xiazhuang and Zhuguang ore fields are located within the Nanling Metallogenic Belt in South China. These deposits are hosted in Triassic and Jurassic granites that were emplaced during the Indosinian orogeny and the Yanshanian post-orogenic extension. Four main types of granitic host rocks were identified based on geochemical characteristics as the most favourable sources of U for the mineralisation in the district: (i-ii) Indosinian peraluminous S-type (Maofeng) and L-type leucogranite (Baishuizhai); (iii) Indosinian highly fractionated high-K calc-alkaline A2-type granite (Xiazhuang, Maofeng and Youdong) and (iv) early Yanshanian highly fractionated high-K, (Fe) calc-alkaline A2-type granite (Sundong, Siqian and Changjiang). They contain high U concentrations ranging from 15 to 28 ppm indicating the crystallisation of uraninite, which is a source of U easily leachable by oxidised fluids for the formation of hydrothermal mineralisation.This study characterises the mineralogical, chemical and isotopic signatures of the U mineralisation from five representative deposits and occurrences in the Xiazhuang (Baishuizhai, Shituling, Xiwang and Xianshi) and Zhuguang (Mianhuakeng) ore fields. The 175 ± 16 Ma Baishuizhai occurrence and 162 ± 27 Ma Shituling deposit represent an early stage of U mineralisation (175–145 Ma) that occurred during the early Yanshanian. Disseminated Th-bearing uraninite at Baishuizhai and the association of U oxides with alteration minerals such as epidote and chlorite in both Baishuizhai and Shituling indicate temperatures >250 °C for the hydrothermal system. The early Yanshanian granites (190–150 Ma) provided the heat source and magmatic fluids to the hydrothermal system while U-rich Indosinian granites provided U for the Zr-Th-Ta-bearing U mineralisation. In contrast, the mineralisation from Xiwang, Mianhuakeng and Xianshi deposits (107 ± 16, 93 ± 15 and 79 ± 11 Ma, respectively) represent several pulses of the main mineralisation stage (110–50 Ma) that occurred during the late Yanshanian crustal extension. The mineralisation occurred at relatively low temperatures (<250 °C) as W-Y-Nb-Ti-rich pitchblende and uraninite veins associated with quartz, fluorite, calcite and pyrite. During Cretaceous-early Cenozoic crustal extension, oxidising meteoric and/or basinal waters percolated downward into the granitic basement through deep faults and mixed with CO2-rich magmatic fluids. These thermal solutions circulated along opening fractures and leached U from Indosinian and early Yanshanian U-rich granites and also possibly from early Yanshanian U mineralisation. After heating, the ore-forming fluids ascended along structures leading to the decrease of the hydrostatic fluid pressure, which promoted the deposition of U in veins. Therefore, granite-related hydrothermal U mineralisation in the province, formed over a long time span (>100 Ma) in two major stages related to different genetic conditions.

Geochemistry and geochronology of the Banxi Sb deposit: Implications for fluid origin and the evolution of Sb mineralization in central-western Hunan, South China

Central-western Hunan in South China hosts the largest antimony belt in the world with two types of Sb deposits identified: Sb-Au Woxi-type and Sb only Xikuangshan-type. Banxi is the most representative deposit in the region with vein-type Sb mineralization hosted in Neoproterozoic clastic rocks, stibnite developing in ores, and arsenopyrite mainly occurring in altered country rocks.Trace element contents (including rare earth elements; REE) and isotopic ratios (S, Pb, Sr, Nd, He and Ar) in stibnite and/or arsenopyrite were analyzed to determine the timing of ore formation and to elucidate fluid origin and evolution processes. Most sulfides have high ΣREE contents (45–103ppm), moderate fractionation between LREE and HREE (LREE/HREE=6.38–11.56) and slightly negative Eu anomalies (Eu/Eu*=0.53–0.72). The δ34S values are 3.88–5.81‰ for stibnite and 9.25–11.82‰ for arsenopyrite. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios range from 18.617 to 18.635, 15.695 to 15.739, and 38.965 to 38.981 for stibnite, and from 18.594 to 18.609, from 15.587 to 15.698, and from 38.893 to 38.926 for arsenopyrite. The 87Rb/86Sr and 87Sr/86Sr ratios are 0.3016–3.538 and 0.711463–0.717591, respectively for stibnite, and 0.2251–2.214 and 0.711244–0.711565, respectively for arsenopyrite. The 147Sm/144Nd and 143Nd/144Nd ratios are 0.1174–0.9816 and 0.511942–0.512768 for stibnite, with εNd(t) values (t=130Ma) ranging from −12.4 to −6.6, and two-stage model ages (T2DM) ranging from 1457 to 1932Ma. The 40Ar/36Ar and R/Ra ratios (where Ra is the atmospheric 3He/4He ratio of 1.4×10−6) are 409–545 and 0.0088–0.0348, respectively for stibnite. The Rb–Sr and Sm–Nd isotopic analyses of the sulfides yield isochron ages of 129.4±2.4Ma (2σ, MSWD=1.3) and 130.4±1.9Ma (2σ, MSWD=1.6), respectively.The combined trace element and isotopic data-set indicates that the ore-forming fluids were a mixture of dominantly deep basement-derived, solute-rich parent water and a small amount of dilute, heated meteoric water. The decreases in ΣREE, δ34S, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 87Rb/86Sr, 87Sr/86Sr, 147Sm/144Nd and 143Nd/144Nd and increases in 40Ar/36Ar and 4He contents in the stibnite from the earlier quartz-stibnite mineralization stage (Stage II) through to the later stibnite mineralization stage (Stage III) indicate that the influx of meteoric water increased during Sb mineralization. The deep-sourced, relatively high temperature, rock-reacted fluid mixed with increasing amounts of more oxidized, low temperature meteoric fluids, causing the precipitation of arsenopyrite and stibnite sequentially at the Banxi deposit.A two-period genetic model has been proposed for the world-class Sb mineralization in the central-western Hunan region. The Woxi-type Sb–Au deposits were derived from orogenic fluids generated from folding and shearing during the Caledonian and Indosinian Orogeny, whereas the Xikuangshan-type Sb deposits were formed by mixing heated basement-derived hydrothermal fluid circulating during the flare-up of magmatism in South China, with lower temperature meteoric water.

A middle Triassic extensional event in the Hainan Island: Geochronologic and geochemical evidence from igneous rocks from Dazhou Island

Hainan Island lies at the southern tip of the South China block that was affected by the opening and closure of the paleo-Tethys from Paleozoic to Mesozoic and subsequently by the subduction of the paleo- Pacific plate beneath the Euroasian plate since the late Mesozoic era. This study presents zircon U-Pb ages and geochemistry of granites and basic diabase veins in Dazhou Island located 5 km southeast of Hainan Island. The age for granites is 237.2 Ma (MSWD = 0.14, n = 14), suggesting that they were emplaced in the late middle Triassic and are products of magmatic-tectonic episodes during the closure of paleo-Tethys. The granites belong to the shoshonitic magmatic series and are metaluminous I-type granites. They are systematically depleted in Ba, Nb, Sr, P, and Ti, and have variable ranges of initial 87Sr/86Sr ratios (0.71005 ~ 0.71110), but relatively constant eNd(t) values (–6.4~–6.8). The two-stage depleted mantle Nd model ages (TDM2 = 1531–1562 Ma) for these granites suggest that they may have originated from partial melting of the lower crust with a crustal residence age of early Middle Proterozoic Era. The zircon U-Pb ages for diabase veins can be divided into three groups. The age for Group I is 236.8 Ma (n = 4), which represents the crystallization age of the veins and somewhat younger than the accompanying granites. The ages for Group II and III are 1767 Ma (n = 6) and 2432 Ma (n = 3), respectively. These suggest that the veins have inherited some old zircons that possibly represent old geological events in the Hainan Islands. The diabase also belongs to the shoshonitic magmatic series and has arc-like trace element characteristics. The veins have high 87Sr/86Sr and low 143Nd/144Nd ratios, which suggest that they may have originated from an enriched mantle II type source or a mantle source metasomatized by aqueous fluids released from previously subducted slab. The new ages obtained for the Dazhou igneous rocks in the present study are similar to those for the Qiongzhong batholith in the Hainan Island, which implies that the Hainan Island had also regionally experienced a middle Triassic extension event after the Indosinian Orogeny.

Stratigraphy of deformed Permian carbonate reefs in Saraburi Province, Thailand

Kilometre-scale thrusts affect the stratigraphic order of the sedimentary rocks that form the Khao Khwang Platform in central Thailand. The effect of these thrusts requires detailed analysis to unravel the original distribution of the sedimentary units and their associated facies. The Khao Khwang Platform comprises three carbonate-dominated units identified previously as the Phu Phe, Khao Khad and Khao Khwang Formations. These carbonates are intercalated with clastic, mixed siliciclastic and carbonate sequences: the Sap Bon, Pang Asok and Nong Pong Formations. The palaeogeography of the area prior to the Indosinian Orogeny is poorly known and has herein been investigated by combining structural reconstructions, microfacies and biostratigraphy in the Saraburi area. A stratigraphic model for the Yakhtashian–Midian carbonate platform palaeogeography using the well-developed exposed sections of the Khao Khwang Platform, located in the Saraburi Province of Thailand, is presented. This has revealed the presence of several separate carbonate platforms dominated by four major Middle Permian facies (peritidal, platform interior, algal reef and basin slope), which are dated using foraminifera and algae. This model integrates recent structural studies and considers their impact on the present-day distribution of the facies in terms of platform development using biostratigraphy and detailed microfacies analysis. Supplementary material: Characteristic images of the six facies, high-resolution stratigraphic logs, a timescale correlation chart and a table detailing the common fossils identified in this study are available at https://doi.org/10.6084/m9.figshare.c.3861796 .

Structural analysis of the Baolun gold deposit, Hainan Island, South China: Implications for metallogeny

The Baolun gold deposit is a mesothermal orogenic gold deposit located in the southwestern part of Hainan Island, South China. The deposit comprises a series of NNW-trending quartz-sulfide lodes situated within a parallel array of fault zones traversing a sequence of variably foliated flysch siliciclastic rocks of the Lower Silurian. Detailed field mapping documented at least five phases of deformation in the deposit including NNW-trending folding of the Lower Silurian rocks (D1), development of NNW-trending, steeply dipping ductile shear zones with an oblique dextral sense corresponding to NNE-SSW shortening (D2), WNW-ESE shortening and extension associated with an early oblique sinistral ductile shearing along the NNW-trending fault zones (D3), ENE-WSW shortening (D4), and near N-S extension (D5). The gold-bearing quartz lodes cut the strata folded in the D1, show some laminar layering related to ductile shear in the D2 and are overprinted by brittle structures formed in the D3 to D5. 40Ar–39Ar dating on muscovite from an auriferous quartz lode yielded an age of 242±2.5Ma, which, together with the age of 232±2.5Ma for an aplite vein in the deposit, suggests that the mineralization may be related to a tectono-thermal event in the Triassic. In the context of the southern South China plate tectonics, the formation of the Baolun gold deposit is interpreted to be related to the oblique dextral ductile shearing (D2) along the NNW-trending fault zones during the Indosinian orogeny, in relation to the convergence between the Indochina and South China plates.

Late Cretaceous extension and exhumation of the Stong and Taku magmatic and metamorphic complexes, NE Peninsular Malaysia

Fragmentation of large continental areas by post-orogenic extension requires favourable geodynamic conditions and frequently occurs along pre-existing suture zones or nappe contacts, as exemplified by the Stong and Taku magmatic and metamorphic complexes of northern Peninsular Malaysia. For this case, we have employed a field and microstructural kinematic study combined with low temperature thermo-chronology to analyse the tectonic and exhumation history. The results show that the Late Palaeozoic - Triassic Indosinian orogeny created successive phases of burial related metamorphism, shearing and contractional deformation. This orogenic structure was subsequently dismembered during a Cretaceous thermal event that culminated in the formation of a large scale Late Santonian - Early Maastrichtian extensional detachment, genetically associated with crustal melting, the emplacement of syn-kinematic plutons and widespread migmatisation. The emplacement of these magmatic rocks led to an array of simultaneously formed structures that document deformation conditions over a wide temperature range, represented by amphibolite- and greenschist- facies mylonites and as well as brittle structures, such as cataclastic zones and normal faults that formed during exhumation in the footwall of the detachment. The formation of this detachment and a first phase of Late Cretaceous cooling was followed by renewed Eocene - Oligocene exhumation, as evidenced from our fission track ages. We infer that an initial Cretaceous thermal anomaly was responsible for the formation of an extensional gneiss dome associated with simple shear and rotation of normal faults. These Cretaceous processes played a critical role in the establishment of the presently observed crustal structure of Peninsular Malaysia.

U–Pb and Re–Os geochronology and geochemistry of the Donggebi Mo deposit, Eastern Tianshan, NW China: Insights into mineralization and tectonic setting

The Donggebi Mo deposit located in NW China is a newly discovered, large, stockwork-type Mo deposit with ore reserves of 441Mt @ 0.115%Mo. Ore bodies occur along faults and fractures at the external contact zone of a concealed porphyritic granite and volcaniclastic rocks of Gandun Formation, spatially associated with a fine-grained granite. Mo-bearing veins are mainly assemblages of volatile-rich K-feldspar-quartz-oxide, K-feldspar-quartz, polymetallic sulfides and calcite-quartz. Zircon LA-ICP-MS U–Pb dating yielded concordant ages of 234.6±2.7Ma and 231.8±2.4Ma for the porphyritic granite and the fine-grained granite, respectively; molybdenite Re–Os dating gave an isochron age of 234.0±2.0Ma. These ages further confirm an important and extensive magmatic-metallogenic event in Eastern Tianshan during the Triassic Indosinian orogeny. Whole-rock major and trace element analyses indicate that the granitic rocks associated with Mo mineralization are high in Si, K, Rb, Th, Nb, Ta, Ga and LREE, but low in P, Ti, Sr and Ba, belonging to high-K calc-alkaline granites with A-type features. Magma was likely derived from the re-melting of thickened lower crust in a post-collision compression environment in the Late Permian, experienced strong crystal fractionation and formed the large Donggebi Mo deposit under an intra-plate extension setting in the Early to Middle Triassic.

Gneiss domes of the Danba Metamorphic Complex, Songpan Ganze, eastern Tibet

In this paper we address the formation and exhumation of the Danba Metamorphic Complex (DMC) that represents the deepest structural level of the Songpan Ganze terrane situated along the eastern margin of the Tibetan plateau. The DMC comprises a variety of gneiss domes and offers a unique opportunity to decipher their development during orogenic evolution. For that purpose, PTtD paths of metamorphic rocks sampled at different structural levels have been reconstructed. The DMC is composed of Triassic metaturbidites of the Xikang group, Paleozoic metasedimentary cover and basement of the Yangtze craton. The DMC is structurally marked by transposition of the upright S1 foliation of the Triassic metaturbidites into a NW-SE trending S2 composite foliation dipping to the NE. Transposition is associated with a localized top-to-the-northeast shear zone along the northeastern edge of the DMC and with pervasive top-to-the-southwest shearing from the core to the border of the complex. These structures are consistent with extrusion of the core of the DMC relative to the lower grade Triassic metaturbidites. The position of the biotite isograd overlapping the structural boundary of the DMC suggests that the Triassic metaturbidites have been affected by an increase in temperature as a result of extrusion. Within the DMC, the position of the metamorphic index minerals relative to the composite S2 foliation reveals that biotite, garnet, staurolite and kyanite grew before the transposition into S2, in contrast with sillimanite which crystallizes in the hinge of F2 folds and along the axial planar S2 schistosity. The sillimanite isograd delineates regional-scale overturned F2 folds and cross-cuts the staurolite and kyanite isograds consistent with an increase in temperature during D2. The melt-in isograd characterizes the deepest structural level of the DMC. PT conditions for these metamorphic rocks, determined using pseudosections and conventional thermometry, indicate a temperature increase from 400°C to more than 600°C from the edge to the core of the DMC for a relatively homogeneous pressure ranging from 5 to 6.5kbar suggesting that isograds and isotherms represent the syn-D2 thermal structure of the orogenic crust. Migmatites exposed in the deepest structural level of the DMC yield a pressure significantly lower than the surrounding metamorphic rock suggesting that they crystallized after D2 and after some exhumation of their hosts. Three different types of gneiss domes are distinguished on the basis of their position relative to the isograds, their structural characteristics, and their position relative to the margin of the Yangtze craton. Close to the craton and at the highest structural level, the Gezong dome represents a basement-rooted tectonic slice, in an intermediate position, the Gongcai dome corresponds to a basement-cored nappe, and further away and at the deepest structural level, the Bawang, Cunuchan and Qingaling domes are migmatite-cored domes. The presence at the current-day surface of this variety of gneiss domes reflects the difference in burial of the margin during the Mesozoic Indosinian orogeny.

Palaeostress magnitudes in the Khao Khwang fold-thrust belt, new insights into the tectonic evolution of the Indosinian orogeny in central Thailand

Using the Khao Khwang fold–thrust belt in central Thailand as case study, we handled data from calcite twinning analysis in order to propose the quantification of the effective principal palaeostresses magnitudes since the onset of the early stages of the Indosinian orogeny. We also combined the differential stress estimates from mechanically-induced calcite twins with geochronological data in order to constrain the timing of the palaeoburial depth and subsequent uplift by folding within the Khao Khwang fold–thrust belt. The proposed mechanical scenario is based on the time-constrained kinematic sequence of fracturing, faulting, and folding in the strata of the carbonate formations of the Saraburi Group. Cross-checking the data on the palaeostress orientations, regimes, and differential stress magnitudes with rock mechanics analysis; we provided data on the principal stress magnitudes for each tectonic stage that developed during the Indosinian orogeny. Further, 40Ar/39Ar and U-Pb geochronological data allowed to place reliable constraints on the amount and rate of vertical uplift of the carbonate formations of the Saraburi Group.

New geochronology constraints on timing and depth of the ancient earthquakes along the Longmen Shan fault belt, eastern Tibet

Pseudotachylyte is an ideal target to directly date ancient earthquake associated with regional faulting. Here we perform step-heating 40Ar/39Ar, clay mineral K-Ar, and zircon fission track (ZFT) analyses on the pseudotachylyte samples collected from the Yingxiu-Beichuan co-seismic rupture of the Longmen Shan fault belt (LSFB), to provide time constraints for the tectonic evolution of the LSFB during the Indosinian orogeny. 40Ar/39Ar results from the matrix show that the frictional melting occurred 226-235 Ma ago. Combined with mylonite dating of the host rock, the age of the ancient earthquakes is constrained at 231-238 Ma, with a formation depth of 10-14 km. As a response to the earthquakes, a series of soft-sediment deformation structures are widely preserved in the Middle and Late Triassic strata along the Yingxiu-Beichuan fault (YBF), indicating that the LSFB was seismically active since the late Anisian and persisted episodically until the end of the Indosinian orogeny. Both clay mineral K-Ar and ZFT analyses record a younger deformation or alteration with an age of ca. 195 Ma, corresponding to a post-collisional orogeny. These new data represents the first direct evidence of the regional thrusting (YBF) in the central LSFB during the Indosinian orogeny, concurrent with the initial ductile deformation of the western boundary fault. Tectonic inheritance then strongly influenced the evolution of the LSFB as most of the Mesozoic faults are reactivated by major Tertiary tectonic deformations.

Late Paleogene rifting along the Malay Peninsula thickened crust

Sedimentary basins often develop above internal zones of former orogenic belts. We hereafter consider the Malay Peninsula (Western Sunda) as a crustal high separating two regions of stretched continental crust; the Andaman/Malacca basins in the western side and the Thai/Malay basins in the east. Several stages of rifting have been documented thanks to extensive geophysical exploration. However, little is known on the correlation between offshore rifted basins and the onshore continental core. In this paper, we explore through mapping and seismic data, how these structures reactivate pre-existing Mesozoic basement heterogeneities.The continental core appears to be relatively undeformed after the Triassic Indosinian orogeny. The thick crustal mega-horst is bounded by complex shear zones (Ranong, Klong Marui and Main Range Batholith Fault Zones) initiated during the Late Cretaceous/Early Paleogene during a thick-skin transpressional deformation and later reactivated in the Late Paleogene. The extension is localized on the sides of this crustal backbone along a strip where earlier Late Cretaceous deformation is well expressed. To the west, the continental shelf is underlain by three major crustal steps which correspond to wide crustal-scale tilted blocks bounded by deep rooted counter regional normal faults (Mergui Basin). To the east, some pronounced rift systems are also present, with large tilted blocks (Western Thai, Songkhla and Chumphon basins) which may reflect large crustal boudins. In the central domain, the extension is limited to isolated narrow N-S half grabens developed on a thick continental crust, controlled by shallow rooted normal faults, which develop often at the contact between granitoids and the host-rocks. The outer limits of the areas affected by the crustal boudinage mark the boundary towards the large and deeper Andaman basin in the west and the Malay and Pattani basins in the east.At a regional scale, the rifted basins resemble N-S en-echelon structures along large NW-SE shear bands. The rifting is accommodated by large low angle normal faults (LANF) running along crustal morphostructures such as broad folds and Mesozoic batholiths. The deep Andaman, Malay and Pattani basins seem to sit on weaker crust inherited from Gondwana-derived continental blocks (Burma, Sibumasu, and Indochina). The set of narrow elongated basins in the core of the Region (Khien Sa, Krabi, and Malacca basins) suffered from a relatively lesser extension.

Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis

The Indosinian orogeny marks the termination of marine deposition and the accumulation of lower Permian to Late Triassic clastic sediments in the Youjiang Basin, South China Block. Major and trace element compositions of Early to Middle Triassic sedimentary clastic rocks from Youjiang Basin were analysed to constrain their provenance and tectonic setting. Argillaceous samples have low SiO2 (average 56.95wt.%), Al2O3 (average 15.15wt.%), and Fe2O3T+MgO (average 11.54wt.%) contents, and high K2O/Na2O (average 15.61) and Al2O3/SiO2 (average 0.27) ratios, similar to mudstones from continental arc basins. Arenaceous samples have moderate SiO2 (average 76.98wt.%), Al2O3 (average 8.41wt.%), and Fe2O3T+MgO (average 5.29wt.%) contents, and moderate Al2O3/SiO2 (average 0.11) and K2O/Na2O (average 15.26) ratios, identical to those of graywackes from continental island arcs or active continental margins. Both the argillaceous and arenaceous samples have low CIA values (57–85) and relatively high ICV values (0.69–2.11), indicating that the source rocks experienced weak chemical weathering and the sedimentary detritus was derived from an immature source. Compared with late Permian to Early Triassic South China granitoids and upper crust, the samples have lower contents of high-field-strength elements (e.g., Zr, Hf, Nb, and Ta) and large ion lithophile elements (e.g., Rb, Sr, Ba, Th, U, and Pb). However, their relatively high Rb concentrations (>51ppm), and low Rb/Sr (0.16–4.19) and Th/U (2.66–5.21) ratios, are indicative of an igneous source from a continental arc that underwent weak chemical weathering. Both the argillaceous and arenaceous samples are moderately enriched in light rare earth elements and show relatively flat chondrite-normalized heavy rare earth element patterns (LaN/YbN=6.61–17.35; average 10.61) with strong negative Eu anomalies (Eu/Eu*=0.54–0.89; average 0.73). In tectonic discrimination diagrams, including Th–Sc–Zr/10 and La–Th–Sc plots, the geochemical data suggest that the clastic rocks were deposited in a continental arc or margin setting. Thus, we infer that Early to Middle Triassic sediments in the Youjiang foreland basin record the transition from late Permian and Early Triassic subduction to Middle Triassic collision at the southwestern margin of the South China Block.

The giant South China Mesozoic low-temperature metallogenic domain: Reviews and a new geodynamic model

The South China Craton was formed by amalgamation of the Yangtze and Cathaysia Blocks during the Neoproterozoic. During the Mesozoic, voluminous granitic plutons and associated W-Sn polymetallic deposits were formed in the Cathaysia Block. The giant South China low-temperature metallogenic domain (LTMD) includes an area of ∼500,000km2 in the Yangtze Block and is composed of the Chuan-Dian-Qian Pb-Zn, Youjiang Au-As-Sb-Hg and Xiangzhong Sb-Au metallogenic provinces. The Chuan-Dian-Qian Pb-Zn province contains numerous MVT Pb-Zn deposits, whereas the other two provinces are characterized by Carlin-type Au deposits and vein-type Sb, Hg and As deposits. These epigenetic deposits, which formed under low temperature conditions (∼100–250°C), are typically hosted in sedimentary rocks and are locally controlled by faults and fractures. The deposits formed dominantly at 200–230Ma and 130–160Ma, corresponding to Indosinian (Triassic) and Yanshanian (Jurassic to Cretaceous) orogenies, respectively. Indosinian mineralization is recognized in all three provinces, but Yanshanian mineralization occurred only in the Youjiang and Xiangzhong provinces. The Indosinian orogeny, which involved collision of the Indochina Block with the South China Craton, resulted in circulation of basinal brines that leached ore-forming elements from adjacent sedimentary strata to form the Chuan-Dian-Qian Pb-Zn province. Deep-seated granitic magmas generated during this orogeny caused extensive circulation of meteoric water that mobilized ore-forming elements from the sedimentary strata to form the Carlin-type Au deposits in the Youjiang province, and the Sb-Au deposits in the Xiangzhong province. The Indosinian orogeny was the key factor in establishing the metallogenic framework of the LTMD. It produced widespread mineralization in the three metallogenic provinces, each of which has unique features reflecting differences in the nature and composition of the basement rocks. The Yanshanian metallogeny was less important and overprinted the older ore deposits in the Youjiang and Xiangzhong provinces.

Demise of Late Triassic sponge mounds along the northwestern margin of the Yangtze Block, South China: Related to the Carnian Pluvial Phase?

Upper Triassic (Carnian) marine successions in the northwestern margin of Upper Yangtze Region (Sichuan Basin, China) show a lithological change from grey oolitic into a sponge-mound limestone (Units 1 and 2 of the lower member of the Ma'antang Formation), then overlain by greyish black to dark grey sandy shale and siltstone (Units 3 and 4 of the upper member of the Ma'antang Formation). Siliceous sponge mounds were built by Hexactinellida, and this succession was examined in three localities: Jushui section in Anxian (JS), and Guanyinya (HWG) and Qingyangou sections (HWQ) in Hanwang, Mianzhu. The conodont Quadralella polygnathiformis confirms a Carnian age for the biolithite and oolitic limestone of the lower Ma'antang Formation. Abundant ammonoids identified as belonging to the Discotropitid and Juvavitid families suggest a Tuvalian 1 (early Late Carnian) age for the lowermost part of the greyish black sandy shale that overlies the sponge mound at the Jushui Section. Field investigations and microfacies analysis suggest that sponge mounds had two stages of growth in relatively deeper water with low energy. The first sponge-growth mound stage ended in a regional karstified omission surface, and was followed by a sudden increase of siliciclastic input. Greyish black shales containing plant fossils cover a second Upper Carnian sponge mound stage, which is mainly recorded in HWG. The onset of these shales may be related to the Carnian Pluvial Phase (CPP) documented in the western Tethys region (e.g., in Italy, Austria and Hungary). The demise of the sponge mounds at all three sections may have been triggered by the joint effect of climatic changes associated with the fresh water input caused by CPP and with a relative sea-level change caused by local tectonic movements in the course of the Indosinian orogeny.

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Mesozoic gold mineralization in Hainan Province of South China: Genetic types, geological characteristics and geodynamic settings

Numerous gold deposits and occurrences in Hainan Province of South China, predominantly of Mesozoic age account for more than 143t of proven gold reserves. The Au mineralization occurs either along a group of NE-, NNW- and WNW-trending shear zones or within NW- to NNW-trending intraformational detachment faults which are closely related to folding, shearing and transpressional to transtensional deformation. Combined with the Mesozoic tectonics and associated magmatism, the ore geology, fluid inclusion geochemistry, C-H-O-S-(Pb) isotopes and geochronology consistently indicate that there are at least two gold deposit-types in Hainan Island, i.e., orogenic-type and intrusion-related.The predominant orogenic gold mineralization, which produces more than 95% of gold metal reserves in Hainan Island, formed in the Early Mesozoic (ca. 228–224Ma). This ore deposit-type, represented by the Baolun, Gezhen, and Wangxia deposits, is generally hosted by metamorphosed volcaniclastic sedimentary rocks of the Paleo- to Mesoproterozoic, the Silurian and the Permian ages, and is closely associated with brittle-ductile shearing. These deposits, with native gold as main gold occurrence and low sulfide abundance (<5% by volume), are derived from the CO2-rich (mainly 4.8–16.8mol%), near neutral (pH=∼7), low-salinity (generally 3–10.5wt.% NaCleq.), and intermediate-temperature ore fluids. In combination with the paleomagnetism, sedimentation, and petrographical and geochemical features of the Late Paleozoic to Early Mesozoic granitoids, the orogenic gold mineralization is considered to be formed in a post-collisional tectonic setting, in response to the Indosinian orogeny in South China triggered by the closure of the Paleotethys Ocean. Abundant Bi-As-Te-Mo-S phases, which grew synchronously with the gold minerals, suggest a possible involvement of magmatic fluids, especially for the large-scale, high-grade Baolun deposit.The Fuwen Au-dominated Au-Ag deposit, which is hosted by Lower Cretaceous continental clastic rocks, was interpreted as an intrusion-related deposit and most likely formed in the Late Cretaceous. This deposit is endowed with high gold grade (average 28–95g/t Au), extremely high sulfide abundance (>50% by volume), Au-Ag-Cu-Pb-Zn metal association, and small amounts of alteration minerals (pyrite, quartz, sericite, chlorite and calcite). The rare isotopic data of O-H-S-Pb, and the ore occurrences mainly in the intraformational detachment fault zones and subordinately within the Late Cretaceous adakite-like granitoids (zircon U-Pb age of ca. 100Ma), suggest that the Fuwen deposit had an intimate genetic link to the arc-related extension-type magmatism, due to asthenospheric upwelling triggered by the slab roll-back of the subducted Paleo-Pacific plate beneath the South China Block.