The petrology and geochemistry of intrusive alkaline rocks of Elchuru, Prakasam District, Andhra Pradesh, India
作者:
S. Nag,
P. S. Chakravorty,
T. E. Smith,
C. H. Huang,
期刊:
Geological Journal
(WILEY Available online 1984)
卷期:
Volume 19,
issue 1
页码: 57-76
ISSN:0072-1050
年代: 1984
DOI:10.1002/gj.3350190105
出版商: John Wiley&Sons Ltd
关键词: Alkaline complex (miaskitic);Alkaline gneiss;Ijolite‐melteigite;Malignite Nepheline syenites;(hypersolvus and subsolvus);Ti‐augite;Aegirine‐augite;Major element;Trace element;XRF Magmatic differentiation
数据来源: WILEY
摘要:
AbstractThe Elchuru alkaline igneous intrusion is an arcuate‐shaped ring complex, approximately 16 km2in area, cropping out in the Eastern Ghats high grade metamorphic series. It is part of an alkaline province composed of a number of intrusions which range from ijolite‐melteigite to alkali gabbro (viz. malignite, melalusitanite, shonkinite) and then to hypersolvus nepheline syenite followed by subsolvus nepheline syenite. The complex is cut by late lamprophyric dykes. A nephelinized alkaline gneiss, within the investigated complex, is the only deformed rock type and is regarded as an older unit not related to the comagmatic series. The remainder of the complex was emplaced post‐tectonically. Sovitic carbonatite is a conspicuous Iithologic unit associated with the complex.Chemical analyses of 19 selected samples for 13 major oxides and 5 trace elements (Rb, Ba, Sr, Zr and Nb) are presented to establish a geochemical model for the investigated complex.The mineralogy, petrography and geochemistry of the rocks of the Elchuru Alkaline Complex suggest that it was formed by differentiation of an initially alkali‐rich ijolitic magma as reflected in the crystallization of nepheline, kaersutitic amphibole and alkali feldspar. With progressive increase in alkali feldspar content (volume percentage) the ijolite passes to malignite and then nepheline syenites. Amphibole shows sodic enrichment from a dominant calcic variety. Pyroxene, likewise, shows sodic enrichment following the scheme salite‐ferrosalite to aegirine‐augite. The igneous cycle closes with the intrusion of biotite lamprophyre.There is a systematic increase in total alkalies (Na2O+K2O) and decrease in CaO from the early mafic rocks to the syenitic rocks. The alkali‐lime index of the complex is 48 indicating its strongly alkaline nature (Peacock 1931), and they are miaskitic in character (agpaicity index<1, Currie 1976). Such miaskitic complexes are associated with carbonatites (H
点击下载:
PDF
(1402KB)
返 回