Garnet exsolution in garnet clinopyroxenite and clinopyroxenite xenoliths in early Cretaceous intrusions from the Xuzhou region, eastern China
Garnet exsolution in garnet clinopyroxenite and clinopyroxenite xenoliths in early Cretaceous intrusions from the Xuzhou region, eastern China
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DOI：
10.1180/0026461046830198
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发表时间：
2004-06-01
影响因子：
2.7
通讯作者：
Wang, D
中科院分区：
文献类型：
--
作者：
Xu, W;Liu, X;Wang, D
徐州地区早白垩世闪长质-二长闪长质侵入体中的石榴子石和单斜辉石捕虏体中富含铝质单斜辉石，有时与出溶角闪石和闪长角闪岩共生。除矿物出溶体外，原生单斜辉石颗粒周围还存在粒间细粒石榴石、锆石、钛铁矿，有时还存在角闪石。单斜辉石显示成分梯度，减少铝和增加硅和镁，相邻的石榴石片晶和粒间石榴石。与此相反，出溶和晶间石榴石是均匀的组合物，通常更富钙铝榴石和镁铝榴石贫比粗粒原生石榴石。对前体组合和门网出溶体的P和T的估算表明，温度从950 - 1100 ℃降至620 - 780 ℃，压力从15 - 20 kbar降至18 kbar，与相关榴辉岩的变质条件相当。因此，石榴单辉石岩和单辉石岩捕虏体中的石榴子石出溶可能是高压变质作用的结果。榴辉岩和片麻岩捕虏体的SHRIMP锆石U-Pb定年和同一榴辉岩的Sm-Nd全岩-石榴石定年表明，华北克拉通东南缘部分下地壳可能参与了三叠纪高压变质作用。
Exsolved garnet, sometimes associated with exsolved amphibole and zoisite, is abundant in aluminous clinopyroxene of garnet clinopyroxenite and clinopyroxenite xenoliths in the early Cretaceous dioritic-monzodioritic intrusions from the Xuzhou region, similar to100 kill west of the Sulu ultrahigh-pressure (UHP) metamorphic terrane in eastern China. In addition to the mineral exsolutions, intergranular fine-grained garnet, zoisite, titanite, and sometimes amphibole are also present around the primary clinopyroxene grains. Clinopyroxene shows compositional gradients, with decreasing Al and increasing Si and Mg, adjacent to the garnet lamellae and intergranular garnet. In contrast, the exsolved and intergranular garnet is homogeneous in composition and typically more grossullar-rich and pyrope- and almandine-poor than the coarse-grained primary garnet. The estimates of P and T for the precursor assemblage and gat-net exsolution suggests a decrease of temperature from similar to950-1100degreesC down to 620-780degreesC, and a nearly constant pressure from similar to15-20 kbar to 18 kbar, which are comparable with the metamorphic conditions of the associated eclogites. Therefore, garnet exsolution in garnet clinopyroxenite and clinopyroxenite xenoliths could be caused by high-pressure (HP) meta morphism. SHRIMP zircon U-Pb dating for the associated eclogite and gneiss xenoliths and Sm-Nd whole rock-garnet dating for the same eclogite demonstrate that parts of lower crust oil the southeastern margin of the North China craton might be involved in the Triassic HP metamorphism due to the deep subduction of the Yangtze craton.