GEOLOGICAL AND PETROGRAPHICAL CHARACTERISTICS OF SORAP MASSIF GABBROS, RAS-KOH OPHIOLITE, BALOCHISTAN, WESTERN PAKISTAN

This study discussed the Sorap Massif which is the ophiolitic fragment composed of the upper mantle and lower crustal section of an ophiolitic sequence. An ophiolitic sequence in Sorap Massif consists of harzburgite, dunite, large distant units of serpentinized wehrlite intrusion and outcrop of confined layered gabbro covered by the Quaternary sand dunes. The basal part of gabbroic intrusion is in contact with mantle rocks and the upper part is juxtaposed with the Kuchakki Volcanic Group. On outcrop-level and in hand specimen, the gabbros exhibit needle-like ferromagnesian minerals including hornblende associated with plagioclase and pyroxene. Petrographically the gabbros are classified into norite, gabbro norite and gabbro. The mineral constituents of norite are plagioclase, orthopyroxene and amphibole, gabbro norite is consists of equal constituents of orthopyroxene and clinopyroxene, plagioclase and amphibole and the gabbro is composed of clinopyroxene, plagioclase and amphibole. The high constituents of minerals such as pyroxene, amphibole and low constituents of olivine in the Sorap gabbros indicates that these gabbros are formed by the immature part of the oceanic plate with dehydration of the oceanic plate subduction.

The origin of the c. 1.7 Ga gabbroic intrusion in the Hekou area, SW China: constraints from SIMS U–Pb zircon geochronology and elemental and Nd isotopic geochemistry

The late Palaeoproterozoic to early Mesoproterozoic igneous rocks of southwestern China are characterized by a number of mafic intrusions and dykes. However, the origin and tectonic implications of these mafic intrusions and dykes remain unclear. The Hekou mafic intrusion, intruding into the Hekou Group in the Hekou area, SW China, is the biggest and most representative one. The intrusion is mainly composed of coarse-grained in the central zone (CZ) and medium- to fine-grained gabbroic rocks in the outer zone (OZ). Cameca secondary ion mass spectroscopy (SIMS) U–Pb zircon ages, and geochemical and Nd isotopic results for the intrusion are reported in this paper. SIMS U–Pb zircon ages indicate that the gabbroic rocks from the CZ and OZ were emplaced at 1735±6.5 Ma and 1736±4.0 Ma, respectively. This suggests that the Hekou intrusion originated from c. 1.7 Ga mafic magmatism in the southwestern Yangtze Block. The coarse-grained rocks in the CZ of the intrusion show fairly homogeneous major- and trace-element compositions. In contrast, the medium- to fine-grained rocks from the OZ display slightly evolved compositions, with relatively lower Mg nos, MgO, Al2O3, Cr and Ni contents, and higher SiO2, CaO and Zr concentrations than those of the rocks from the CZ. Although the gabbroic rocks of the intrusion have low total rare earth element (REE) contents (REE = 29.3–40.2 ppm) with slightly light REE (LREE)-enriched and heavy REE (HREE)-depleted patterns, they exhibit distinct trace-element and Nd isotopic features. The rocks from the CZ are characterized by slightly LREE-enriched and ‘convex upwards’ incompatible trace-element patterns with significant Th depletion and insignificant Nb and Ta depletion relative to La. However, the rocks from the OZ have relatively flatter REE patterns than those of the rocks from the CZ. In addition, the rocks from the OZ are slightly enriched in Th and depleted in Nb and Ta relative to La. The εNd(T) values of the CZ and the OZ rocks are +0.70 to +2.3 and −0.30 to +0.24, respectively. The parental magma for the Hekou gabbroic intrusion exhibits affinity with a subalkaline basaltic magma, which was possibly generated by relatively high degrees of partial melting of a slightly depleted asthenospheric mantle source. Their geochemical and isotopic variations were due to slight crystal fractionation with varying degrees of crustal contamination. The Hekou intrusion was therefore supposed to form in an anorogenic extensional environment. It is further suggested that c. 1.7 Ga is an important onset timing of widespread anorogenic magmatism in the southwestern Yangtze Block. We interpret the late Palaeoproterozoic gabbroic intrusion to represent anorogenic mafic magmatism, which was most likely related to the break-up of the Columbia supercontinent.