徐淮地区镁铁质岩石捕虏体单斜辉石中石榴石和石英的出溶

Garnet and Quartz Exsolutions in Clinopyroxenes of Mafic Rock Xenoliths from Xuzhou-Northern Anhui Province

徐淮地区早侏罗世侵入杂岩中榴辉岩、石榴辉石岩和单斜辉石岩捕虏体单斜辉石中可以观察到丰富的出溶石英针和石榴石、黝帘石及角闪石的出溶叶片。榴辉岩中出溶石英针的绿辉石核部比其边部相对富含FeO和MgO ,贫SiO2 、Al2 O3 和CaO。在石榴辉石岩和单斜辉石岩捕虏体中具有出溶石榴石的单斜辉石 ,从靠近出溶石榴石的一侧向其核部 ,Al2 O3 、Na2 O和TiO2 含量降低 ,MgO、SiO2 和CaO含量增加。单斜辉石中定向石英针的出溶表明曾经存在有超高压条件下 (≥2 5× 10 8Pa)稳定的过硅质绿辉石。单斜辉石中出溶石榴石表明温压条件的降低可能是引起出溶的一个主要原因。捕虏体中的矿物组合和岩相学特征表明它们曾经遭受了榴辉岩相和角闪岩相退化变质作用 ,这与因压力和温度降低引起矿物出溶的结果相吻合。

There occurs an Early Jurassic intrusive complex belt in Xuzhou-northern Anhui province which is distributed in NE direction parallel to Tan Lu Fault. It is mainly composed of diorite and monzodiorite. The 40 Ar/ 39 Ar plateau ages of hornblendes from diorite and monzodiorite are in the range of 189～191Ma. The eclogite, garnet pyroxenite and clinopyroxenite xenoliths are found in this intrusive complex. Abundant exsolved quartz rods and exsolution lamellae of garnet are identified within or between clinopyroxenes of eclogite, garnet clinopyroxenite and clinopyroxenite xenoliths under microscope. The core of the omphacite, in which quartz rods occur, is relatively poor in SiO 2, Al 2O 3, CaO, but rich in FeO and MgO than its rim. There is also a systematic compositional change within the clinopyroxenes containing exsolved garnets: the core is obviously rich in Al 2O 3, Na 2O and TiO 2, and the rim in contact with exsolved garnet is rich in MgO, SiO 2 and CaO. The garnets exsolved within clinopyroxene are similar to those exsolved between clinopyroxenes in mineral chemistry, which indicates their exsolution origin. A spectacular microstructure of oriented quartz rods within clinopyroxene implies preexistence of silica oversaturated omphacite stabile at ultrahigh pressure (≥25kbar) conditions. The existence of exsolved garnet within single clinopyroxene crystal indicates that decrease of temperature and pressure would be a major cause of exsolution. Therefore, we conclude that most exsolution microstructures in the xenoliths may have been resulted from the decomposition of very high pressure precursor phases (clinopyroxene) due to decrease of pressure and temperature (especially pressure). The mineral assemblage and petrography of the xenoliths show that the xenoliths were subjected to eclogite facies metamorphism and amphibolite facies retrogressive metamorphism, which is consistent with mineral exsolution due to decrease of pressure. The discovery of high pressure xenoliths and mineral exsolution will be helpful for us to understand the subduction and exhumation history of Dabie Sulu UHP metamorphic belt.