Regresar

Episodic mantle melting-crustal reworking in the late neoarchean of the Northwestern North China Craton: Zircon ages of magmatic and metamorphic rocks from the Yinshan Block

Abstract:

The Yinshan Block in the northwestern North China Craton comprises late Neoarchean greenstones, high-grade metamorphic rocks (granulite, charnockite, enderbite and amphibole gneiss), diorites (sanukitoid and Mg-adakite) and granitoids (predominantly trondhjemite-tonalite). A greenstone terrane comprises a volcanic sequence of predominant basalt and dacite, with minor andesite. Zircons from a high-Mg andesite and an andesite have weighted mean 207Pb/206Pb ages of 2533±5Ma and 2510±7Ma, respectively, which reflect the timing of two distinct volcanic episodes. These two either predated or postdated a major volcanic episode that formed a basalt-dacite bimodal suite during 2515±10 (basalt) to 2516±10Ma (dacite). Magmatic zircons from the high-grade metamorphic rocks yielded weighted mean 207Pb/206Pb ages of 2544±5Ma for a mafic granulite, 2521±6Ma for a garnetiferous gneiss, 2535±5Ma for a charnockite, and 2526±5Ma for an amphibole gneiss. These ages reflect the (minimum) emplacement time of rocks that make up the protoliths of the high-grade metamorphic complex. The metamorphic ages of the high-grade complex were determined at 2515±18Ma for mafic granulite and 2503±12Ma for garnetiferous gneiss. Zircons from a high-Mg diorite (i.e. sanukitoid) have a weighted mean age of 2520±6Ma, which is significantly younger than the 2556±14Ma age of a magnesian diorite (Mg-adakite) from the same diorite massif. These ages reflect distinctive magmatic pulses that generated the diorite massif. A tonalite has a zircon age of 2516±7Ma, indistinguishable from the 2520±9Ma age of a trondhjemite from the same trondhjemite/tonalite-dominated batholith. A granitic compositional layer in the 2516±7Ma tonalite has zircon ages of 2515±6Ma (high-U zircon domains) and 2512±22Ma (low-U zircon domains), which reflect an intracrustal melting event. The presence of an older crustal remnant (ca. >2600Ma) prior to the formation of the Yinshan Block is indicated by a 2594±4Ma leucogranite xenolith in the diorite massif and by the widespread presence of ca. 2583-3485Ma (mostly, ca. 2700Ma) zircon xenocrysts in a variety of volcanic and plutonic rocks. An undeformed pegmatite dike in high-Mg andesite has an emplacement age of 2508±17Ma (zircon). An undeformed trondhjemite dike that cuts the supracrustal greenstone sequence yielded a zircon age of 2502±6Ma, similar to that of the pegmatite dike, and representing the latest Archean magmatic event in the Yinshan Block. These ages of undeformed granitoid dikes delimit the termination of thermo-tectonic development of the Yinshan Block. Based on these new ages and published geological and geochronological data, we conclude that the Yinshan Block is a typical Neoarchean cratonic block, not a segment of the so-called "Paleoproterozoic Inner Mongolia-North Hebei Orogen". Episodic mantle melting and resultant crustal reworking account for the thermo-tectonic development of the Yinshan Block in a brief period (ca. 2556-2502Ma) in the late Neoarchean, and based on their regional chronological correlations we argue for their craton-scale significance. © 2012 Elsevier B.V.