Origin of large plagiogranite bodies and their relationship to massive sulfide ore deposits in the Semail Ophiolite, northern Oman
Plagiogranite intrusions on the scale of tens of meters to kilometers are exposed as dikes, sills, and small stocks in the Semail ophiolite in northern Oman, mainly within the high-level gabbro and the sheeted dike complex. They are composed primarily of quartz, sodic plagioclase, diopsidic augite, amphibole, epidote, and sphene, and have been interpreted as products of extensive crystal fractionation of mid-ocean ridge basalt magma and/or partial melting of hydrated mafic roof rocks. Some of the smaller concordant bodies may have formed at or near the original spreading center, but most of plagiogranites crosscut the primary ophiolite stratigraphy. These off-axis intrusions are commonly zoned, with ultramafic rocks and gabbros lying beneath the plagiogranites. Petrographic, microprobe, and oxygen isotope studies have been carried out on about 210 samples from seven of the largest bodies and 10 of the smaller bodies, together with studies of the adjacent country rocks. Most of the bodies contain myrmekitic intergrowths of quartz, epidote, and albite coexisting with relict enclaves similar to compositions in the host gabbros and mafic dikes. Mafic enclaves with multiple origins, as dismembered late mafic dikes, net-veined mafic pillows, and mafic hornfels xenoliths can comprise up to 50% of the plagiogranite intrusions. These features are interpreted as artifacts of injection of coeval mafic magmas into the silicic magmas, accompanied by variable degrees of assimilation of hydrothermally altered roof rocks (some with δ^(18)O as low as +3 to +4). δ^(18)O values of plagiogranite quartz are heterogeneous, compatible with models of partial melting and/or variable degrees of crustal assimilation. Coexisting plagioclase is invariably out of isotopic equilibrium with quartz as a result of post-crystallization alteration by heated seawater. Zones of intense alteration at the upper contacts of the plagiogranites include albite-chlorite-epidote-sphene-actinolite-sulfide, and these grade into the chlorite-sulfide-jasper stockwork zones of the economic sulfide deposits (as at Lasail). These ^(18)O-depleted, epidote-rich aureoles are associated with pervasive replacement of calcic plagioclase by albite and epidote, pyroxene by amphibole, epidote, or chlorite, and Fe-oxide by sphene. Heat from these off-axis intrusions and metals derived from the hydrothermal metamorphism are responsible for the formation of the overlying sulfide deposits.
© 1994 Geological Society of America.