Petrogenesis and implications for tectonic setting of Cambrian suprasubduction-zone ophiolitic rocks in the central Beishan orogenic collage, Northwest China. (1st December 2015)

• Record Type: Journal Article
• Authors: Cleven, Nathan; Lin, Shoufa; Guilmette, Carl; Xiao, Wenjiao; Davis, Bill
• Is Part Of: Journal of Asian earth sciences. Volume 113:Part 1(2015)
• Page Start: 369
• Page End: 390
• Publication Date: 2015-12-01
• Subjects: Central Asian Orogenic Belt -- Volcanic arc -- Ophiolite -- Geochemistry -- Spinel mineral chemistry -- U–Pb geochronology; Earth sciences -- Asia -- Periodicals; Sciences de la terre -- Asie -- Périodiques; Earth sciences; Asia; Periodicals; 555.05
• DOI: 10.1016/j.jseaes.2014.10.038
• Languages: English
• ISSNs: 1367-9120

Abstract

Highlights: The Hongliuhe ophiolite recorded Cambrian active seafloor spreading (520.3 Ma). Ultramafics comprise tectonized cumulate-layered lherzolite and gabbro lithologies. Spinel chemistry shows a forearc affinity partially re-equilibrated with intrusions. Bimodal volcanic suite has island-arc tholeiite and calc-alkaline geochemistries. Supracrustal covering of nearshore oceanic sedimentary rocks and volcaniclastics. Abstract: The Hongliuhe ophiolite is a fragment of the early Paleozoic oceanic crust situated in a suture zone central to the Beishan orogenic collage, which is a subset to the Central Asian Orogenic Belt in Northwest China. It has lithologic, geochemical and chronological similarities to the Yueyashan–Xichangjing ophiolite 400 km east, along strike, allowing for correlative interpretations of marginal seafloor spreading processes. The Hongliuhe ophiolite is explored in this study with complete petrologic, geochemical and structural descriptions. The ultramafic body comprises cumulate-layered lherzolite and gabbro plutonic lithologies, tectonized with syn-magmatic extension. A U–Pb age of 520.3 ± 5.8 Ma was obtained by SHRIMP analysis of zircons from a pegmatitic gabbro. The ultramafic rocks are variably depleted and some show refertilization of LILE's consistent with slab dehydration in a supra-subduction mantle wedge. Mineral chemistry shows high Cr# (36–82) populations of spinel, and wide-ranging lower Mg# (8–50), consistent with known forearc peridotite chemistry partially re-equilibrated by tectonism or through staged magmatism. Olivine and pyroxene chemistries are consistent with this interpretation. A thin volcanic cover is interlayered with red cherts, and has a bimodal set of geochemical signatures, both island-arc tholeiite and calc-alkaline. They bear distinct Nb–Ta anomalies consistent with slab-derived fluid initiated melting in a supra-subduction environment. Coarse volcaniclastic turbidites infilled paleo-topography along the ophiolite, and they are interbedded with quartzofeldspathic sandstones, indicating an initial proximity to an arc. The sandstones contain detrital spinel with analyses showing relation to both the ophiolite and possibly a secondary alkalic volcanic source. The supracrustal sequence continues with a thick, contiguous covering of margin-related rocks, including interlayered limestone, pyroclastic and siliciclastic rocks, and basalts. As a constraint on emplacement we provide an U–Pb Zircon SHRIMP date (413.6 ± 3.5 Ma) from an undeformed granitoid body intruded within the overlying stratigraphy. The entire analysis paints a picture of subduction rollback-related extension affecting the margin of an emerging or mature arc just prior to a known magmatic cycle that ended with a collision in the Devonian at the latest. This suture is one of many in the Beishan, and our interpretation provides evidence in a long running debate supporting accretionary orogenesis in an island-arc archipelago as the main system for growing sialic crust in the Paleozoic central Asia.