New Evidence of an Ultra-Deep Continental Subduction to Mantle Depth (~300 km) in Stishovite Stability Field
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摘要: 南阿尔金高压-超高压变质岩带泥质片麻岩中发现先存斯石英出溶蓝晶石+尖晶石的显微结构证据，将陆壳深俯冲的深度推进到了斯石英稳定域的地幔深度（≥ 300 km）.然而，该类岩石是局部出现的还是具有一定的普遍性以及又是如何折返出露地表的?十多年来一直困惑着地球科学界.针对这一科学问题，通过系统的岩石学研究，在南阿尔金榴辉岩中首次发现了斯石英副象，重新厘定南阿尔金英格利萨依石榴辉石岩中石榴子石出溶单斜辉石和北秦岭松树沟长英质片麻岩中石榴石出溶石英棒状体岩石的峰期变质压力为9~10 GPa的斯石英稳定域，结合先期南阿尔金泥质片麻岩中发现先存斯石英出溶显微结构的研究成果，论证提出陆壳俯冲到斯石英稳定域的地幔深度（~300 km），然后再折返回地表的地质现象可能更为普遍，其岩石类型也可能具有多样性.通过高温高压实验研究，明确SiO2饱和岩石体系中石榴子石超硅的最小稳定条件为≥ 9~10 GPa斯石英稳定域，为识别辨认陆壳岩石俯冲到斯石英稳定域地幔深度的研究提供了新的借鉴和思路.Abstract: The discovery of kyanite+spinel exsolutions in former stishovite of pelitic gneisses from the South Altyn HP-UHP metamorphic belt suggests an ultra-deep subduction and exhumation of the South Altyn continental rocks to/from mantle depths in the stability field of stishovite (≥ 300 km). Whether such an ultra-deep subducted rock is specific sample or common case is still enigmas to geologists for more than a decade. To solve this problem,through a series of petrological studies,it is found for the first time quartz paramorphs after stishovite in omphacite and garnet of eclogite from Jianggalesayi,South Altyn UHP belt,and the peak metamorphic pressures of Yinggelisayi garnet pyroxenite with clinopyroxene exsolutions in garnet and Songshugou felsic gneiss with quartz exsolutions in garnet are re-estimated to be ≥ 9-10 GPa in the stability field of stishovite. These lines of new evidence,together with the previous discovery of kyanite+spinel exsolutions in the former stishovite of the pelitic gneiss from the South Altyn UHP belt,suggest that ultra-deep subduction of continental materials might be more common and diverse than previous thought. The minimum stability pressure of super-silicon garnet in SiO2-saturated rock system is estimated to be ≥ 9-10 GPa based on high temperature and high pressure experiments,which provides a new method to confirm the ultra-deep subduction of continental materials to mantle depth in stishovite stability field (~300 km).
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图 1 南阿尔金榴辉岩中先存斯石英副象的显微结构
据Liu et al.(2018)；Qmp.绿辉石；Grt.石榴子石；Qz.石英
Fig. 1. Microstructures of the paramorphs of the former stishovite of eclogite in South Altyn Tagh
图 2 南阿尔金石榴子石辉石岩中石榴子石出溶单斜辉石的显微结构
Cpx.单斜辉石; Liu et al.(2005)
Fig. 2. The microstructure of garnet dissolved clinopyroxene of garnet pyroxenite in South Altyn Tagh
图 3 北秦岭松树沟长英质片麻岩中石榴子石出溶金红石(a)、磷灰石(b)和石英+金红石(c，d)的显微结构
据Liu et al.(2003)；Grt.石榴子石；Rut.金红石；Ap.磷灰石；Qz.石英
Fig. 3. Microstructures of garnet dissolved rutile (a), apatite (b) and quartz+rutile (c, d) of Songshugou felsic geniss in North Qinling
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