Alpine and Pacific styles of Phanerozoic mountain building: subduction‐zone petrogenesis of continental crust

Abstract

AbstractA broad continuum exists between two distinct end‐member types of mountain building. Alpine‐type orogenic belts develop during subduction of an ocean basin between two continental blocks, resulting in collision. They are characterized by an imbricate sequence of oceanward verging nappes; some Alpine belts exhibit superimposed late‐stage backthrusting. Sediments are chiefly platform carbonates and siliciclastics, in some cases associated with minor amounts of bimodal volcanics; pre‐existing granitic gneisses and related continental rocks constitute an autochthonous–parautochthonous basement. Metamorphism of deeply subducted portions of the orogen ranges from relatively high‐pressure (HP) to ultrahigh‐pressure (UHP). Calcalkaline volcanic–plutonic rocks are rare, and have peraluminous, S‐type bulk compositions. In contrast, Pacific‐type orogens develop within and landward from long‐sustained oceanic subduction zones. They consist of an outboard oceanic trench–accretionary prism, and an inboard continental margin–island arc. The oceanic assemblage consists of first‐cycle, in‐part mélanged volcaniclastics, and minor but widespread cherts ± deep‐water carbonates, intimately mixed with disaggregated ophiolites. The section recrystallized under HP conditions. Recumbent fold vergence is oceanward. A massive, slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable, non‐subducted plate. It consists of abundant, dominantly intermediate, metaluminous, I‐type volcanics resting on old crust; both assemblages are thrown into open folds, intruded by comagmatic I‐type granitoids, and metamorphosed locally to regionally under high‐T, low‐P conditions. In the subduction channel of collisional and outboard Circumpacific terranes, combined extension above and subduction below allows buoyancy‐driven ascent of ductile, thin‐aspect ratio slices of HP–UHP complexes to midcrustal levels, where most closely approached neutral buoyancy; exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume. At massive sialic buildups associated with convergent plate cuSPS (syntaxes), tectonic aneurysms may help transport HP–UHP complexes from mid‐ to upper‐crustal levels. The closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts, so volcanic–plutonic belts are poorly developed in Alpine orogens compared with Circumpacific convergent plate junctions. Generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation. Phase equilibrium studies show that, under typical subduction‐zone P–T trajectories, clinoamphibole ± Ca–Al hydrous silicates constitute the major hydroxyl‐bearing phases in deep‐seated metamorphic rocks of MORB composition; other hydrous minerals are of minor abundance. Ca and Na clinoamphiboles dehydrate at pressures of above approximately 2 GPa, but low‐temperature devolatilization may be delayed by pressure overstepping; thus metabasaltic blueschists and amphibolites expel H2O at melt‐generation depths, and commonly achieve stable eclogitic assemblages. Partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions. For reasonable subduction‐zone geothermal gradients however, white micas ± biotites remain stable to pressures >3 GPa. Accordingly, attending descent to depths of >100 km, mica‐rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of H2O, and transform incompletely to stable eclogite‐facies assemblages. Underflow of amphibolitized oceanic lithosphere thus generates most of the deep‐seated volatile flux, and the consequent partial melting to produce the calcalkaline suite, along and above a subduction zone; where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths, volatile flux severely diminishes. Thus, continental collision in general does not produce a volcanic–plutonic arc whereas in contrast, the long‐continued contemporaneous underflow of oceanic lithosphere does.