The Jijal Complex, Pakistani Himalayas: An ultramafic-mafic intrusion from the crust-mantle boundary
Abstract
The Jijal Complex and the overlying metamorphosed layered intrusions represent an oblique section through the crust of an accreted island arc. As such, it provides a window into crustal structure and chemistry unmatched anywhere else in the world. This research was undertaken in order to present the first rigorous and systematic mapping and sampling study of this region. Petrographic and geochemical analyses allowed identification of this complex as part of an island arc. These data also established that the complex was not a metamorphosed garnet granulite as previously believed, but rather a pristine igneous layered intrusion with minor metamorphic character. Geothermometry and geobarometry, based on thermodynamic data and mineral analyses, demonstrates a smooth trend of decreasing pressure up section that is geophysically relevant. The intrusion crystallized near the Moho, at a depth of 35 to 45 km, and a pressure of 12.5 to 14.5 kilobars, from bottom to top. Near economic grades of precious metals were sampled from several horizons, suggesting more detailed investigations into the economic potential of the area are warranted. The complex has been characterized as a layered, ultramafic-mafic intrusion, emplaced at or very near the crust mantle boundary, that has subsequently been exhumed to its present location. The parental magma must have been in equilibrium with mantle peridotite, with MgO $>$ 16 wt. %. These magmas must have been hydrous as well, since they saturated with hornblende so early in the fractionation sequence. The liquids responsible for precipitating the bulk of the mafic rock must have been tonalitic. Crustal seismic structure corroborates the interpretation that the Kohistan province is an accreted island arc. In general, the Kohistan arc has a remarkably similar seismic structure to other collisional orogenic terranes, and may well serve as a ground-truth model for remote sensing generated models in island and continental arcs.
Degree
Ph.D.
Advisors
Levandowski, Purdue University.
Subject Area
Geology|Geochemistry|Geophysics
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