Genesis of zoned hydrous ultramafic/mafic-silicic intrusive complexes: an MHFC hypothesis

Abstract

It is proposed that hydrous ultramafic and mafic rocks can originate from the hydration of water-depleted mafic magmas by coexisting water-enriched silicic magmas. This would cause hornblende and biotite to directly crystallise from the hydrated melts, while hydration reactions would result in widespread replacement of existing anhydrous mafic minerals (Ol, Opx and Cpx) by hornblende and biotite. Chemical and mineralogical zonation in ultramafic/mafic-silicic intrusive complexes can thus result from three closely related processes: magma mixing, hydration reactions and fractional crystallisation (MHFC). Although not all hydrous ultramafic/mafic rocks are formed by this mechanism, the MHFC hypothesis may give an alternative explanation for the origin of hydrous ultramafic/mafic rocks in those complexes where mixing of a water-depleted mafic magma and a water-enriched silicic magma can be demonstrated. Evidence supporting this hypothesis includes: (1) contact relations; (2) net-veining and mutual intrusions of coexisting mafic and silicic magmas; (3) quenched mafic encalves; (4) skeletal growth and branching of hornblende, plagioclase, titanite and acicular apatite indicative of quenching of a high-temperature mafic magma against a relatively low-temperature silicic magma; (5) synplutonic hydrous mafic composite dykes; (6) comb layering and orbicular structure; (7) regular concentration of hydrous ultramafic and hydrous mafic rocks along the contract zones between a water-depleted mafic magma and a water-enriched silicic magma; (8) strong zonation of these complexes; (9) coexistence of magmatic hornblende and biotite with those formed by hydration reactions of anhydrous mafic minerals; (10) similar isotopic and geological ages for different rocks ranging from ultramafic to silicic within these complexes; (11) Sm/S, Rb/Sr, and O isotopes indicating that different rocks within these complexes are derived from varying degrees of mixing of a mantle-derived mafic magma with a crust-derived silicic magma. All these observations can be consistently explained by the proposed MHFC model.