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Carboniferous and Cretaceous mafic-ultramafic massifs in Inner Mongolia (China): A SHRIMP zircon and geochemical study of the previously presumed integral "Hegenshan ophiolite"

Abstract:

We present SHRIMP zircon ages and whole-rock geochemical data to identify mafic-ultramafic massifs of early Carboniferous and early Cretaceous ages from northern Inner Mongolia, China. The early Carboniferous massifs (Chongenshan-Xiaobaliang-Wusnihei) comprise predominantly lherzolite and minor gabbroic rocks. Zircons from a microgabbro (ε Nd (t)=9.8) and a plagiogranite (ε Nd (t)=9) yielded weighted mean 206Pb/ 238U ages of 354±7Ma and 333±4Ma, that we interpret as reflecting the time of zircon (and magma) crystallization. A volcanic breccia resting unconformably on the ultramafic rocks (serpentinites) has an eruption age of 300±2Ma and provides a younger minimum time limit for the emplacement of the early Carboniferous mafic-ultramafic massifs. In contrast, the early Cretaceous mafic-ultramafic massifs (Hegenshan-Chaogenshan) consist predominantly of harzburgite. A pegmatoid gabbro and a rodingite (ε Nd (t)=10.6) from the Hegenshan massif have magmatic zircon ages of 139±2Ma and 132±7Ma respectively. The Chaogenshan massif is concentrically zoned with an ultramafic core (dominantly harzburgite-dunite+subordinate lherzolite) surrounded by gabbro (with minor hornblendite) and an outermost volcano-sedimentary sequence (metabasaltic flows+metapelites). We dated zircons from a leuocogabbro (125±2Ma; ε Nd (t)=10.6), a coarse-grained gabbro (142±4Ma), and a metabasalt (134±2Ma). Zircon xenocrysts (ca. 152-492Ma) are ubiquitous in these and other associated rocks (a coarse-grained gabbro and a diabase), as in the Hegenshan rocks (ca. 152-468Ma). The youngest detrital zircon in outer zone garnet schist (i.e. metapelite) has an age of 132±2Ma. Accordingly, we conclude that the northern Inner Mongolia lherzolite-dominant and harzburgite-dominant massifs are temporally and genetically unrelated. Their emplacement is attributed to asthenospheric upwelling and consequent lithospheric extension during periods of orogenic quiescence in the southern Central Asian Orogenic Belt. © 2012 Elsevier B.V.