Yuejun Wang

俯冲大洋板片与地幔如何相互作用及形成弧岩浆岩的过程, 是当今地球科学的关键科学问题. 传统观点认为俯冲板片流体和熔体交代的地幔楔部分熔融形成弧岩浆岩. 近年学者提出板片混杂岩底辟体与弧下地幔物理混合后再部分熔融也可形成弧岩浆岩. 但板片混杂岩底辟体与地幔如何相互作用、相互作用形成何种产物、如何有效识别这些产物, 是大地构造学亟待解决的科学问题. 本文综述了近年有关板片混杂岩底辟作用的研究资料, 列举了板片混杂岩底辟体与弧下地幔物理混合形成弧岩浆岩的相关证据, 并依据数值模拟将板片混杂岩底辟体细分为大洋沉积物底辟、混杂岩底辟和蚀变洋壳底辟等三个过程. 综述发现全岩Sr-Nd-Hf-O等同位素组成及其比值的联合使用可区分板片流体/熔体交代和混杂岩底辟物理混合两种模式. 俯冲大洋沉积物底辟体具低δ30Si和δ138/134Ba值、高εHf(t)和δ18O值、全岩Nd-Hf同位素解耦等特征, 而蚀变洋壳底辟体具有重的Mg、B和Ba同位素及轻的O和Li同位素等. 板片混杂岩与弧下地幔或OIB(洋岛玄武岩)型地幔混合可形成铌钽亏损弧岩浆岩或富铌弧岩浆岩. 板片混杂岩底辟的研究仍存在一些问题静待地质学家深入研究.

Mesozoic igneous rocks from Sumatra provide crucial insights into the temporal‐spatial connection with the Eastern Tethys Domain. This study presents new zircon ages and whole‐rock elemental data for the Late Triassic igneous rocks in West Sumatra. These igneous rocks yield consistent ages of 227–211 Ma. Granitoid and rhyolite exhibit A‐type affinities with high K₂O and FeOt contents, and are depleted in HFSEs. They have similar ε_Nd (t) values of −8.3 to −5.0, ε_Hf (t) values of −8.0 to +1.4, and initial ratios for ²⁰⁶Pb/²⁰⁴Pb (18.71–19.17), ²⁰⁷Pb/²⁰⁴Pb (15.74–15.77) and ²⁰⁸Pb/²⁰⁴Pb (39.28–39.48). They were derived from an “ancient” meta‐mafic source with a juvenile mafic crust component. Mafic‐intermediate igneous rocks can be classified as medium‐ to high‐K calc‐alkaline series and are characterized by depletion in Nb, Ta, and Ti. Mafic‐intermediate volcanic rocks show higher ε_Nd (t) values of −0.7 to +1.3 and ε_Hf (t) values of +3.5 to +4.2 than the diabases with ε_Nd (t) of −9.0 to −8.5 and ε_Hf (t) of −5.5 to −1.2. All these igneous rocks share similar initial ratios for ²⁰⁶Pb/²⁰⁴Pb (18.62–18.84), ²⁰⁷Pb/²⁰⁴Pb (15.68–15.75) and ²⁰⁸Pb/²⁰⁴Pb (38.67–39.05), suggesting a metasomatized lithospheric mantle origin. These Late Triassic igneous rocks share similar isotopic compositions with the synchronous post‐collisional igneous rocks along the Eastern Paleo‐Tethys giant igneous belt and were formed in a post‐collisional extensional setting. Regional stratigraphy and detrital zircon observations indicate that West Sumatra constitutes the southern part of the Indochina‐East Malaya Block, and the Eastern Paleo‐Tethys zone likely extends into Sumatra.

LCT (Li-Cs-Ta) rare-element pegmatites occur as late-stage and highly-fractionated bodies at the margins of regionally zoned granite pegmatite fields. The evolution of the granitic pegmatite system, including its rare-metal metallogeny is often difficult to determine due to complex textures involving variable crystal size, and a heterogeneous chemical composition. The Renli-Chuanziyuan pegmatite field (South China) displays a well-developed regional zonation sequence, involving a core of biotite-, two-mica-, and muscovite-monzogranites that grades outward into microcline (K-zone), microcline-albite (K-Na-zone), albite (Na-zone) and albite-spodumene (Na-Li-zone) pegmatites. Monazite and the Th, Ca-endmember (i.e., cheralite) provide valuable age, rare earth element (REE) geochemical, and Sm-Nd isotopic data for understanding the regional zoning process within the Renli-Chuanziyuan pegmatite. Monazite (from the muscovite monzogranite (MM), and the K-, K-Na-, and Na-zone pegmatites) and cheralite (from the Na-Li-zone pegmatite) have variable compositions and complex internal microtextures. The monazite and cheralite grains contain irregular areas with subtle heterogeneous BSE response along cracks and grain-margins, suggesting that they have experienced alkali-bearing fluid-aided modification. However, these features are rarely seen in monazite from the K-zone pegmatite. Common Pb contamination and/or Pb loss during fluid-aided modification may have disturbed the monazite and cheralite U-Th-Pb isotopic system, due to the differential mobility of U, Th and Pb. The unaltered Na-zone monazite and Na-Li-zone cheralite yielded Th-Pb ages of 140.42 ± 2.30 Ma (2 σ, MSWD = 2.4, n = 14) and 139.58 ± 2.15 Ma (2 σ, MSWD = 2.9, n = 21), respectively. The unaltered MM, K-zone and K-Na-zone monazite yielded ²⁰⁶Pb-²³⁸U ages of 138.03 ± 2.18 (2 σ, MSWD = 2.5, n = 18), 140.39 ± 2.18 (2 σ, MSWD = 3.0, n = 20) and 140.58 ± 2.14 Ma (2 σ, MSWD = 2.0, n = 52), respectively. These ages for the four pegmatite zones are temporally consistent with a syngenetic origin for the magmatic sequence of biotite-, two-mica-, and muscovite-monzogranite and the pegmatite system and rare metal (Li-Nb-Ta-Rb-(Cs)-(Be)) mineralization. The Sm-Nd isotopic analyses of the unaltered monazite and cheralite from the MM and four pegmatite zones yield similar initial Nd isotopic composition with εNd_(t) = -9.9 to -7.9, indicating an identical single source region (i.e., the Neoproterozoic South China lower crust). The Sm/Nd ratios display a gradual decrease across the four pegmatite zones from the unaltered K-zone monazite to Na-Li-zone cheralite, i.e., 0.39 – 0.63 (avg. = 0.43) for K-zone, 0.29 – 0.35 (avg. = 0.31) for K-Na-zone, 0.26 – 0.30 (avg. = 0.28) for Na-zone, and 0.21 – 0.27 (avg. = 0.24) for Na-Li-zone. Such progressive variations suggest their derivation from the same parental magma, which experienced varying degrees of fractionation prior to the extraction of pegmatitic melts. Comprehensive monazite and cheralite geochemistry, as well as in-situ U-Th-Pb and Sm-Nd isotopic results indicate that Rayleigh-type fractional crystallization controls the mineralogical and geochemical evolution from a chemically zoned granite source.

The 1,900‐km‐long Sumatran fault plays an important role in accommodating oblique plate motion between the Indo‐Australian and Sunda plates. Although fault geometry, kinematics, and seismic activity are well‐studied, the onset of dextral motion on the Sumatran fault and uplift of adjacent Barisan Mountains is unclear; this hinders an understanding of the late Cenozoic evolution of Sumatra and the forearc region. In this study, we use low‐temperature thermochronology to measure cooling histories of rocks within and outside of the Sumatran fault. An accelerated exhumation within the fault zone began at ∼2 Ma. The Barisan Mountains may have experienced uplift and associated river incision in the late Miocene to Pliocene. The fault systems in the forearc region were inferred to be kinematically linked to the Andaman Sea and Sunda Strait and accommodated early relative plate convergence, then relocated the strike‐slip component of deformation to the Sumatran fault at ∼2 Ma.

厘定东亚大陆边缘中生代弧前岩浆记录以限定其俯冲边界及其时序，是解码古太平洋俯冲体系的关键。基于当前具弧前指示意义的基性-超基性岩石较少报道及婆罗洲所处特殊大地构造位置，本文选择菲律宾巴拉望构造带南延之婆罗洲沙巴地区开展了系统研究。对昔加麦火成岩、古达、特鲁比和拉哈达图蛇绿岩基性岩及相关岩石的全岩Ar-Ar和锆石U-Pb年代学、全岩元素和Sr-Nd-Pb同位素地球化学的研究表明：沙巴基底发育有前三叠系岩石，构造上亲缘于华夏陆块或地处巽他古陆东缘。原定义为“沙巴非蛇绿岩的昔加麦基底岩石”的安山岩和英云闪长岩等形成于251-179 Ma，具埃达克质岩石地球化学属性，87Sr/86Sr(i) = 0.70285 ~ 0.70307，εNd(t) = +6.5~+7.7，具太平洋MORB型Pb同位素组成，源自俯冲板片派生组分交代的地幔源区，是古太平洋俯冲体系的重要组成部分。限定出沙巴地区蛇绿岩的形成时代介于早-中侏罗世至晚白垩世(~185-85 Ma)，其中特鲁比蛇绿岩可能较早发育(~185-140 Ma)、古达蛇绿岩主体形成于早白垩世(~135-112 Ma)、而拉哈达图蛇绿岩形成持续至晚白垩世(~85 Ma)。沙巴蛇绿岩中镁质岩石发育具相似Sr-Nd-Pb同位素组成的MORB型、高铌和富铌基性岩，其87Sr/86Sr(i) = 0.70291- 0.70567，εNd(t) = +6.6~+10.2，(206Pb/ 204Pb)i = 17.91-19.00、(207Pb204Pb)i=15.43-15.58和(208Pb204Pb)i = 37.61-38.47，其源区受板片派生熔体的交代改造，形成于古太平洋俯冲弧前背景。该研究限定出东亚大陆边缘的古太平洋西向俯冲至少自早三叠世(~251 Ma)即已启动、并持续至晚白垩世(~85 Ma)，具长寿命(>150 Ma)“多阶段幕式进撤”的安第斯型俯冲特征，其白垩纪俯冲前锋位于现今中国台湾东部、菲律宾西部，经沙巴、延入古晋带、甚至帕朗卡拉亚一线。该地区构造格局直至晚白垩世末期-新生代早期才转换为现今西太平洋俯冲体系。