Shisha Pangma Leucogranite, South Tibetan Himalaya: Field Relations, Geochemistry, Age, Origin, and Emplacement
Shisha Pangma Leucogranite, South Tibetan Himalaya: Field Relations, Geochemistry, Age, Origin, and Emplacement
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DOI：
10.1086/515924
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发表时间：
1997-05
期刊：
影响因子：
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通讯作者：
M. Searle;R. Parrish;R. Parrish;R. Parrish;K. Hodges;K. Hodges;A. Hurford;A. Hurford;M. Ayres-M.-Ay
中科院分区：
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作者：
M. Searle;R. Parrish;R. Parrish;R. Parrish;K. Hodges;K. Hodges;A. Hurford;A. Hurford;M. Ayres-M.-Ay
构成西藏南部Xixabangma（8027米）大部分岩体的Shisha Pangma岩体是20多个较大的浅色花岗岩侵入体之一，标志着喜马拉雅变质核的最高结构水平。该岩体位于石沙庞玛拆离带的正下方，该拆离带是藏南拆离带（STD）系统的一部分，是一条低角度（30°）北倾正断层，将古生代黑色板岩置于硅线石级泥质岩和钙硅酸盐岩石之上。沿下盘含有纤维岩和硅线石副片麻岩的钾长石眼球状片麻岩沿着显示出强烈的内部S-C组构，表明向下向北延伸。Shisha Pangma浅色花岗岩是一个多相异质侵入体，早期为富含黑云母的片理化相，晚期为富含电气石+白云母的相，通常包含组合：Kfs + Pl + Qtz + Ms + Tur ± Gt ± Bt ± Sil ± Ap。高过铝质花岗岩具有高的87 Sr/86 Sr比值（0.738 - 0.750），这是泥质岩衍生的深熔岩的典型特征。贫钕地幔模型年龄（根据目前的Nd同位素数据和假设的地壳147 Sm/144 Nd为0.10 ± 0.02）为1.5 - 2.2 Ga，表明大部分源物质的地壳驻留年龄为早元古代或更老。STD（X8）正下方弱面理化黑云母花岗岩中的异时代岩和独居石给出了一致的U-Pb年龄，为20.2 ± 0.2 Ma。来自主要石沙庞马电气石+白云母±石榴石相（X20）的锆石、晶质铀矿和独居石的U-Pb年龄为17.3 ± 0.2 Ma。浅色花岗岩主体上方的岩床杂岩平行于变质组构排列，倾角为10 - 30± N，尽管有一些岩脉横切STD下方的变质组构。无色花岗岩没有切割STD，正断层的年龄必须在17.3 ± 0.2 Ma之后。白云母的40 Ar/39 Ar坪年龄为16.74 ± 0.22Ma。5800 - 8000米浅色花岗岩样品的磷灰石裂变径迹年龄范围为12.3 ± 1.9至14.8 ± 0.8 Ma（± 2ρ），仅略小于主要浅色花岗岩结晶年龄。在地壳熔融之后，17 - 14 Ma的陡峭冷却曲线（>90 - 180°C/百万年）和快速折返速率（104 mm/年）导致至少12 km的覆盖层被侵蚀和正常断层移动。如果高侵蚀和折返率与高地形（和高降水）相关，这些数据表明，喜马拉雅山在17 Ma左右达到其最大地形海拔。
The Shisha Pangma pluton forming most of the Xixabangma (8027 m) massif in south Tibet is one of the 20+ larger leucogranite intrusives that mark the highest structural levels of the Himalayan metamorphic core. The pluton occurs immediately below the Shisha Pangma Detachment, a strand of the South Tibetan Detachment (STD) system, a low angle (30°) north‐dipping normal fault placing Paleozoic black slates atop sillimanite‐grade pelites and calc‐silicate rocks. K‐feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal S‐C fabrics indicative of down‐to‐the‐north extension. The Shisha Pangma leucogranite is a heterogeneous, polyphase intrusion with an earlier, foliated biotite‐rich phase and a later, tourmaline + muscovite rich phase typically containing the assemblage: Kfs + Pl + Qtz + Ms + Tur ± Gt ± Bt ± Sil ± Ap. The highly peraluminous granites have high 87Sr/86Sr ratios (0.738‐0.750) typical of pelite‐derived anatectites. Nd‐depleted mantle model ages (from present Nd isotopic data and an assumed crustal 147Sm/144Nd of 0.10 ± 0.02) are 1.5‐2.2 Ga, indicating a substantial early Proterozoic or older crustal residence age for much of the source material. Xenotimes and monazites from a weakly foliated biotite granite immediately beneath the STD (X8) give consistent U‐Pb ages of 20.2 ± 0.2 Ma. Zircon, uraninite, and monazite from the main Shisha Pangma tourmaline + muscovite ± garnet phase (X20) give an U‐Pb age of 17.3 ± 0.2 Ma. A sill complex above the main leucogranite body is aligned parallel to the metamorphic fabric dipping at 10‐30± N, although a few dikes cross‐cut the metamorphic fabric beneath the STD. Nowhere do the leuco‐granites cut the STD, and the age of normal faulting must largely post‐date 17.3 ± 0.2 Ma. Muscovite from the main leucogranite phase has an 40Ar/39Ar plateau age of 16.74 ± 0.22 Ma. Apatite fission track ages for leucogranite samples from 5800‐8000 m range from 12.3 ± 1.9 to 14.8 ± 0.8 Ma (± 2ρ), only slightly younger than the main leucogranite crystallization age. Following crustal melting, steep cooling curves (>90‐180°C/myr) and rapid exhumation rates (∼ 4 mm/yr) from 17‐14 Ma resulted in removal of at least 12 km of overburden, both by erosion and normal faulting. If high erosion and exhumation rates correlate with high topography (and high precipitation) these data suggest that the Himalaya reached their maximum topographic elevation around 17 Ma.