Petrology and geochemistry of the Dargai Ultramafic Complex, Pakistan
Abstract
Three related geological events were most likely responsible for the emplacement of the Dargai Complex: (a) counter-clockwise rotation of India with respect to Eurasia; (b) collision of the Kohistan Complex with the Indus Tsangpo suture and (c) the change from tensional to compressional environments in association with the India and Eurasia collision. These events caused the emplacement of the Darbai Ultramafic Complex into supracrustal rocks. It is further proposed in this study that the southern boundary fault is the trace of MMT and the northern boundary fault is related to post-Eocene southward thrust. Podiform chromite deposits are sporadically distributed in the Dargai Complex. Relict cumulate textures indicate the primary magmatic nature of the chromites. Structurally the Dargai chromitites are classified as concordant, subcordant and discordant depending on the host rock relationship. Refractory and metallurgical types of chromite deposits occur in the Dargai Complex which indicate ophiolitic origin. The Cr/Cr+Al and Mg/(Mg+Fe$\sp{2+}$) ratios show inversely proportional relations which is often observed in alpine complexes. In the Dargai Complex the Cr$\sb2$O$\sb3$ concentrations vary from 37.5 to 59.8 wt% and are higher than those found in most stratiform complexes. Disseminated nickeliferous ore minerals occur in the Dargai Complex. The most common mineral assemblages involve mixtures of heazlewoodite (Ni$\sb3$S$\sb2)$, awaruite (Ni$\sb3$Fe) and/or pentlandite (Fe, Ni)$\sb9$S$\sb8$. These minerals are commonly associated with serpentinized rocks. The physiochemical conditions related to serpentinization control not only the silicates but the ore minerals as well. This study shows that the Dargai Complex has passed through progressive metamorphism of greenschist facies grade at a mid oceanic ridge (pre-tectonic metamorphism). Retrograde metamorphism is also evidenced by the presence of albite in the gabbroic rocks. Syn tectonic metamorphism is marked by the appearance of antigorite in the ultramafic rocks near the contact with pre-Cambrian metasediments. Whole rock analyses (20) were plotted on discriminant diagrams which show that the Dargai Complex was tholeiitic and corresponded in comparison to that of ophiolites. ACM projection of metaigneous rocks show a composition trend characteristic in magmatic differentiation. Some trace elements (i.e., Cr. and Ni) decrease in concentration with increasing degrees of differentiation from ultramafic to mafic rocks. (Abstract shortened with permission of author.)
Degree
Ph.D.
Advisors
Kullerud, Purdue University.
Subject Area
Geochemistry
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