Eclogite and carpholite‐bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle
Eclogite and carpholite‐bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle
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DOI：
10.1111/j.1525-1314.2007.00713.x
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发表时间：
2007-06
影响因子：
3.4
通讯作者：
S. Song;L. F. Zhang;Y. Niu;C. Wei;J. Liou;G. Shu
中科院分区：
文献类型：
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作者：
S. Song;L. F. Zhang;Y. Niu;C. Wei;J. Liou;G. Shu
低温榴辉岩和榴辉岩相变岩与蛇纹岩和大理岩一起以块体形式出现在源于杂砂岩基质的片理化蓝片岩中;它们形成了中国西北部北祁连洋型缝合带的高压低温（HPLT）俯冲杂岩（混杂岩）。根据矿物组合特征，将白云母榴辉岩（Ⅰ型）和绿帘石榴辉岩（Ⅱ型）区分开来。残余硬铝石和硬铝石假像以包裹体形式出现在这两种榴辉岩的石榴石中。石榴绿辉石-多硅白云母地质温压法测得弱变形榴辉岩的变质条件为460-510 °C和2.20-2.60 GPa，强面理榴辉岩的变质条件为475-500 °C和1.75-1.95 GPa。榴辉岩相变质沉积物包括石榴子石-绿辉石-多硅白云母-蓝闪石片岩和各种含氯辉石的片岩。在一些高镁绿帘石片岩中发现了镁榴辉岩。Grt-Omp-Phn-Gln片岩的P-T估计值为2.60-2.15 GPa和495-540 °C，而Mg‐carpholite片岩的P-T估计值为2.45-2.50 GPa和525-530 °C。矿物组合和P-T估算以及同位素年龄表明，大洋岩石圈以及远洋至半远洋沉积物在460 Ma之前已经俯冲到地幔深处（≥75 km）。蓝片岩相退变质发生在c。454-446 Ma，导致榴辉岩变形和硬铝石在折返过程中脱水。北祁连缝合带榴辉岩和变岩的峰值P-T条件表明存在冷俯冲带梯度（6-7 °C km−1），这种冷俯冲在早古生代将大量的H2O带到地幔深部。
Low‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km−1), and this cold subduction brought a large amount of H2O to the deep mantle in the Early Palaeozoic times.