Ultramafic and mafic magmatism in southwestern Tuva

Abstract

This paper presents the results of the first comprehensive geological, petrographic, mineralogical, geochemical, and geochronological studies of ultramafic and mafic magmatism in the poorly explored southwestern Tuva (evidence from the Birdag and Khayalyg massifs). These massifs intrude ortho- and paraschists, which are presumably Middle Proterozoic. Amphibole gabbros in the Khayalyg massifs contain numerous relict host-rock xenoliths. The massifs are dominated by meso- and leucocratic amphibole gabbros with minor ultramafic (serpentinous plagioclase harzburgites and plagioclase lherzolites) and transitional (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbronorites) rocks. The serpentinous plagioclase harzburgites and plagioclase lherzolites occur in the amphibole gabbros of the Birdag massif as small lenticular bodies. The latter are interpreted as xenoliths of ultramafic restites of earlier protrusion rather than mafic-melt differentiates. The wehrlites, olivine clinopyroxenites, hornblendites, and melanocratic olivine gabbronorites forming the outer zones of the xenogenic bodies of ultramafic restites are considered hybrid rocks. They resulted from the contact reactions of mafic melts and their fluids with the xenogenic bodies of ultramafic restites, which were feldspathized during this interaction. In the gabbros from both massifs, the chondrite-normalized content of MREE and HREE is lower and that of LREE is higher than those in N-MORB. The plagioclase peridotites, wehrlites, and olivine clinopyroxenites forming xenogenic bodies among the amphibole gabbros of the Birdag massif are richer in REE (especially LREE) than the ultramafic restites in ophiolite associations. This is because they were infiltrated by fluids enriched in these elements during their alteration under the influence of later mafic melts. The studies suggest that the Birdag and Khayalyg ultramafic–mafic massifs result from the spatial coexistence of (1) more ancient small protrusions of ultramafic restites, which occurred as allochthons among Middle Proterozoic(?) metamorphic rocks, (2) later gabbroic intrusions (from 494 ± 16 to 450–447.4 ± 5 Ma), and (3) hybrid transitional rocks (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbros) making up contact-reaction zones along their boundaries.