Plagiogranites and Gabbroic Rocks from the Mingora Ophiolitic Mélange, Swat Valley, NW Frontier Province, Pakistan

M. Barbieri, A. Caggianelli, M. R. Di Florio and S. Lorenzoni
Dipartimento di Scienze delia Terra, piazzale A. Moro, I-00185 Roma, Italy
DiTEc Univ. Basilicata, via N. Sauro 85, I-85100 Potenza, Italy
Dipartimento Geomineralogico, via E. Orabona 4, I-70124 Bail, Italy

Abstract: Major, trace element composition and Sr isotopic data were collected for gabbroic rocks, plagiogranites and albitites in the ophiolite assemblage from Swat Valley (NW Frontier Province, Pakistan). Petrographic study revealed that these rocks were subjected to important structural and mineralogical modifications due to greenschist-epidote-amphibolite facies sub-sea-floor metamorphism and to brecciation. On the other hand, the examination of whole rock chemical composition and of chemical trends showed that these rocks were affected by some chemical modifications, concerning especially Na2O, K2O and Rb. The very low contents of HFS (high field strength) and RE elements found in gabbroic rocks and plagiogranites were considered to be a primary magmatic feature pointing in part to their cumulitic nature and in part to an origin from a refractory parental magma. The Sr isotopic data indicate that gabbroic rocks and plagiogranites were subjected to exchange with sea water. The particular chemical features shared by gabbroic rocks and plagiogranites suggested that fractional crystallization was a possible evolution process. In contrast, albitites are characterized by anomalously high contents in HFSE and LREE and by values of the 87Sr/86Sr ratio very close to sea water. These features suggest a more complex origin with respect to gabbroic rocks and plagiogranites.

Keywords: plagiogranites • gabbroic rocks • albitites • ophiolites • Pakistan

Mineralogical Magazine; December 1994 v. 58; no. 393; p. 553-566; DOI: 10.1180/minmag.1994.058.393.03
© 1994, The Mineralogical Society
Mineralogical Society (