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Petrogenesis of the Middle Devonian Gushan diorite pluton on the northern margin of the North China block and its tectonic implications

SHUAN-HONG ZHANG^*,,, YUE ZHAO^*, BIAO SONG and DUN-YI LIU

^* Key Laboratory of Crustal Deformation and Processes, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China
Beijing SHRIMP Center, Chinese Academy of Geological Sciences, Beijing 100037, China

Author for correspondence: tozhangshuanhong{at}163.com

The Gushan diorite pluton, located at the northern margin of the North China block, was emplaced during Middle Devonian times (SHRIMP U–Pb zircon age of 390 ± 5 Ma). Rocks from the pluton are characterized by low SiO₂ and high alkali contents, and they show monzodiorite compositions in a total alkali v. silica (TAS) plot. They exhibit light REE-enrichment, no to slightly positive Eu anomalies, strong depletion in Rb, Th, U, Nb, Ta, P, Zr, Hf and Ti, enrichment in Ba, K and Sr, low contents of Y and Yb, and high Sr/Y ratios. They have a relatively narrow range of isotopic compositions with initial ⁸⁷Sr/⁸⁶Sr ratios of ~ 0.7050, _Nd(T) values of –9.5 to –7.5 and zircon _Hf(T) values from –11.8 to –5.8. These features are remarkably similar to another Middle Devonian intrusion, the Shuiquangou syenitic complex at the northern margin of the North China block. These similarities suggest that the two intrusions probably have a common origin. They were considered to be derived from a type I enriched mantle, ultimately with some involvement of ancient lower crustal components, and were likely emplaced in a back-arc extension environment related to southward subduction of the Palaeo-Asian oceanic plate or during the cessation of the subduction. Aluminium-in-hornblende barometry studies of the Middle Devonian Gushan pluton yielded emplacement depths of about 18 km. Combined with previous geobarometry results on the Carboniferous plutons within the Inner Mongolia Palaeo-uplift on the northern margin of the North China block, it is inferred that the uplift and exhumation of the plutons within the Inner Mongolia Palaeo-uplift during Middle Devonian to Late Carboniferous times were not as distinct as those during Late Carboniferous to Early Jurassic times, and the strong uplift and exhumation of the Inner Mongolia Palaeo-uplift were achieved during Late Carboniferous to Early Jurassic times.

Key Words: diorite • petrogenesis • Devonian • North China block

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