Island Arc-Related, Back-Arc Basinal, And Oceanic-Island
Components Of
The Bela Ophiolite-Melange Complex, Pakistan
Ahmed, Z; Ernst, WG
V H WINSTON SON INC, INTERNATIONAL GEOLOGY REVIEW; pp: 739-763; Vol: 41
King Fahd University of Petroleum & Minerals
http://www.kfupm.edu.sa
Summary
The Bela ophiolite-melange complex (BOMC) of Pakistan is a late Cretaceous
(similar to 66 Ma) suprasubduction-zone composite ophiolite. Its abundant mafic
volcanic and hypabyssal rocks are divisible into three compositionally distinct suitesan ensimatic volcanic island-are (suite 1) dominated by low-Ti (Ti < 7200 ppm)
tholeiites, a back-are basinal assemblage (suite 2) containing high-Ti (>10,000 ppm
Ti) rocks, and high-Ti oceanic-island alkaline basaltic rocks (suite 3). Compared to
suites 1 and 2, suite 3 rocks are high in light rare-earth elements (LREE), Zr, Nb, and
Zr/Y, and low in Cr, Zr/Nb, Y/Nb, and Ti/Nb. In Zr/Y ratios, suite 1 < suite 2 < suite
3. The geographic distribution of the three BOMC suites defines a northern (north of
latitude 26 degrees 50' N) island-are terrane characterized by island-are tholeiites
(IAT) and a southern basinal terrane containing back-are basinal basalts (BABB).
Alkaline basalts apparently represent oceanic seamounts, and occur at isolated sites in
both terranes. Some oceanic-island basalts (OIB) are emplaced in younger sediments
overlying the ophiolite. As exceptions to the general distribution, a few tectonic
blocks of high-Ti basalt crop out in the northern terrane, and a few low-Ti basalts
occur in the southern terrane. Representative massifs with a completely intact
ophiolite stratigraphic sequence include the Lak Baran massif in the are terrane and
the Bora Jhal massif in the basinal terrane. The are terrane contains more extensive
chromite deposits than does the basinal terrane. Are chromites possess higher
Cr/(Cr+Al) than do chromites of the basinal terrane, indicative of more refractory
mantle source rocks and higher degrees of partial melting. Nd-Sr isotopic data on 10
BOMC lithologies support their arc-related nature and derivation from an enriched
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source mantle.
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