Granitic magmas: possible and impossible sources, water contents, and crystallization sequences

Abstract

The calc-alkalic rocks of batholiths or their precursors may be generated in deep continental crust, in subducted oceanic crust, in the mantle wedge above, or in processes involving material from all three sources. For the series gabbro–tonalite–granite, we have phase relationships with excess H_2O to 35 kbar (3500 MPa), and the H_2O-undersaturated liquidus surfaces mapped with contours for H2O contents and with fields for near-liquidus minerals. Isobaric diagrams with low H_2O contents provide grids potentially useful in defining limits for the H_2O content of magmas, based on the sequence of crystallization. Conclusions from the experimental framework include: (1) The H_2O content of large granitic bodies is less than 1.5%. (2) Primary granite magmas can not be derived from the mantle or subducted ocean crust. (3) Primary granite magmas with low H_2O content are generated in the crust, and erupted as rhyolites. (4) Primary tonalite and andesite are not generated from mantle peridotite; the H_2O contents required are unrealistically high. (5) Primary tonalite and andesite are not generated in the crust unless temperatures are significantly higher than those of regional metamorphism. (6) Subducted ocean crust yields magmas with intermediate SiO_2 content, but not primary tonalite and andesite. (7) Batholiths are produced from crustal rocks as a normal consequence of regional metamorphism, with the formation of H_2O-undersaturated granite liquid and mobilized migmatites. Some batholiths receive in addition contributions of material and heat from mantle and subducted ocean crust.

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