Journal of Earth system science | |
Geochemistry and petrogenesis of early Cretaceous sub-alkaline mafic dykes from Swangkre-Rongmil, East Garo Hills, Shillong plateau, northeast India | |
Anup K Sinha11  Rajesh K Srivastava11  | |
[1] Igneous Petrology Laboratory, Department of Geology, Banaras Hindu University, Varanasi 221 005, India.$$ | |
关键词: Swangkre-Rongmil; Shillong plateau; mafic dykes; basaltic-andesite; Kerguelen plume; geochemistry; petrogenesis.; | |
DOI : | |
学科分类:天文学(综合) | |
来源: Indian Academy of Sciences | |
【 摘 要 】
Numerous early Cretaceous maï¬c and alkaline dykes, mostly trending in N-S direction, are emplaced in the Archaean gneissic complex of the Shillong plateau, northeastern India. These dykes are spatially associated with the N-S trending deep-seated Nongchram fault and well exposed around the Swangkre-Rongmil region. The petrological and geochemical characteristics of maï¬c dykes from this area are presented. These maï¬c dykes show very sharp contact with the host rocks and do not show any signature of assimilation with them. Petrographically these maï¬c dykes vary from ï¬ne-grained basalt (samples from the dyke margin) to medium-grained dolerite (samples from the middle of the dyke) having very similar chemical compositions, which may be classiï¬ed as basaltic-andesite/andesite. The geochemical characteristics of these maï¬c dykes suggest that these are genetically related to each other and probably derived from the same parental magma. Although, the high-ï¬eld strength element (+rare-earth elements) compositions disallow the possibility of any crustal involvement in the genesis of these rocks, but Nb/La, La/Ta, and Ba/Ta ratios, and similarities of geochemical characteristics of present samples with the Elan Bank basalts and Rajmahal (Group II) maï¬c dyke samples, suggest minor contamination by assimilation with a small amount of upper crustal material. Chemistry, particularly REE, hints at an alkaline basaltic nature of melt. Trace element modelling suggests that the melt responsible for these maï¬c dykes had undergone extreme differentiation (∼50%) before its emplacement. The basaltic-andesite nature of these rocks may be attributed to this differentiation. Chemistry of these rocks also indicates ∼10–15% melting of the mantle source. The maï¬c dyke samples of the present investigation show very close geochemical similarities with the maï¬c rocks derived from the Kerguelen mantle plume. Perhaps the Swangkre-Rongmil maï¬c dykes are also derived from the Kerguelen mantle plume.
【 授权许可】
Unknown
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