科研之友
微信
新浪微博
Facebook
分享链接
摘要
The Chhotanagpur Gneissic Complex (CGC), bearing imprints of widespread high grade metamorphic and magmatic history since Palaeoproterozoic, represents an integral crustal segment of Eastern Indian Shield. The gabbro-anorthosite intrusives constitute a part of mafic-ultramafic magmatism in the CGC. The study area around Dumka (24 degrees 16%26apos; to 24 degrees 20%26apos;N: 87 degrees 13%26apos; to 87 degrees 22%26apos;E) predominantly comprises of granite gneiss and charnockitic country rocks within which gabbro-anorthosite intrusions occur as lenses. Field relations and structural studies reveal that the country rocks of Dumka have suffered three phases of deformation represented by F-1, F-2 and F-3 folds. The gabbro-anorthosite intrusives maintain a sharp contact with the host rocks, deformed and metamorphosed. Relict igneous layering or primary igneous foliation (S-ig) is recorded where metamorphic overprint is minimal. Mineral phases of gabbro-anorthosite rocks suggest that clinopyroxene compositions from gabrro correspond to diopside and clinoferrosilite, while those from anorthosite are clinoferrosilite. Amphiboles from the gabbro-anorthosite rocks are calcic, and range from ferroan pargasite in gabbro to ferroan pargasitic hornblende in anorthosite. Plagioclase from gabbro and anorthosite belong to bytownite and andesine respectively. Chemical composition of garnet in gabbro is almandine. Thermobarometric estimates for Dumka gabbro-anorthosites correspond to 511 degrees C to 915 degrees C and 5.0-7.5 kb pressure, comparable to that estimated for Bengal Anorthosite (593-795 degrees C, 4.1-7.3 kb). Fractionation trend of plagioclase substantiates a single parental magma in the evolution of Dumka gabbro-anorthosite intrusives.
- 出版日期2012-10
