RAPAKIVI GRANITE MAGMATISM: A GLOBAL REVIEW WITH EMPHASIS ON PETROGENESIS

摘要

Rapakivi granites (A-type granites characterized by the rapakivi texture) occur on all continents and are dated at 0.01 to 2.8 Ga, with a pronounced peak in the Mesoproterozoic. Rapakivi magmatism is characteristically bimodal with contemporaneous silicic and mafic rocks. The former comprise mainly metaluminous/peraluminous biotite + hornblende granites and the latter orthopyrox-ene-bearing gabbroic and anorthositic rocks as well as tholeiitic dikes. Some rapakivi complexes in-clude greisen-, vein-, and skarn-type Sn-polymetallic deposits and Fe oxide-Cu(-U-Au-Ag) deposits. Rapakivi granite magmas are generally considered high-temperature partial melts from the lower continental erust. They are restite-poor and crystallize at low confining pressure and low aetivities of water and, commonly, oxygen. The mafic rocks are derivatives of mafic magmas from the subconti-nental mantle. The alkali feldspar ovoids were probably mantled by plagioclase in response to changes in magma composition, pressure, temperature, and water activity affecting the stabilities of feldspars. Nd isotopic data on the silicic and mafic rocks of rapakivi complexes show initial ratios that indi-cate involvement of both mantle and crustal materials. Rapakivi magmatism is initiated by upwelling and fusion of mantle material resulting in mafic underplating and anatexis of deep continental erust. In the Proterozoic, these processes were probably related to formation of a supercontinent.