Relations of andesites, granites, and derivative sandstones to arc‐Trench tectonics

Abstract

Andesitic volcanogenic sequences, granitic batholith belts, and derivative graywacke‐arkose sedimentary successions are prominent rock assemblages associated with alpinotype peridotite‐gabbro belts and other characteristic tectonic features in orogenic regions or mobile belts where repeated crustal deformation and metamorphism have occurred. Field relations in the circum‐Pacific region indicate that andesitic eruptive suites and granitic intrusive suites are commonly consanguineous and roughly contemporaneous and that they have shed voluminous detritus into coeval graywacke‐arkose belts nearby. Modern systems of oceanic trenches and parallel magmatic arcs are probable analogues of the tectonic settings in which the three related rock assemblages formed.Data on crustal geophysics, trace‐element geochemistry, and strontium‐isotope ratios preclude participation of sialic crust in the generation of andesitic magmas at shallow levels but permit alternative hypotheses of primary partial melts from the mantle, derivative melts differentiated from primary basaltic melts, or melts from oceanic lithosphere slabs descending along inclined seismic zones beneath the volcanic arcs. In Quaternary andesitic suites, areal petrologic variations, particularly in potash content, are consistent tranverse to active volcanic chains regardless of longitudinal variations in crustal thickness. Levels of potash content in different suites correlate well with depths to the inclined seismic zone beneath, although significant scatter of points is apparent. Petrologic data from older andesitic terranes can be used to plot approximate positions and inclinations of paleoseismic zones.The anatectic hypothesis for the origin of magmatic plutons in intrusive batholiths is challenged by apparent comagmatic associations with andesitic eruptives, common sequences of intrusion from mafic to felsic, doubtful presence of suitable geosynclinal roots in some areas, available strontium‐isotope ratios, difficult geothermal inferences, and unexpected episodicity or periodicity of repeated intrusive events that are correlative throughout large longitudinal segments of batholith belts. Consistent positions of batholith belts along the trends of relatively high‐temperature and low‐pressure members of paired metamorphic belts suggest that the granitic plutons were emplaced in the roots of complex volcanoplutonic arcs, and that granitic intrusive magmas may be derived from the same deep sources as andesitic eruptive magmas. Transverse petrologic asymmetry within Mesozoic batholiths of western North America is reminiscent of the similar petrologic asymmetry within Cenozoic volcanic terranes, and may be used to construct speculative paleoseismic zones for the volcanoplutonic arcs whose roots the batholiths may represent.Graywacke and arkose sequences that lie on the Pacific side of andesitic volcanogenic and granitic batholith belts are composed mainly of first‐cycle volcanic and plutonic detritus and commonly form large parts of the relatively low‐temperature and high‐pressure members of paired metamorphic belts. Detritus eroded during and between successive episodes of volcanism and plutonism in the adjacent volcano‐plutonic provenances was deposited in parallel subsiding belts, where it was progressively buried as an inverse record of the successive magmatic increments to the arc regions. The graywacke‐arkose belts commonly include two parallel divisions. Distal facies of strongly deformed trench and continental‐rise deposits were ground against and beneath the seaward flanks of the volcanoplutonic arcs. Proximal facies of more orderly strata were deposited in sediment traps between trenches and arcs in the tectonic position occupied by shelves, slopes, and troughs of varied bathymetric character in modern arc‐trench systems.The interpretations in this paper attempt to bring petrologic inferences about orogenic rock assemblages in line with current mobilist tectonic concepts that are supplanting previous stabilist views. The formation of the three rock assemblages discussed is probably the principal means by which continental crust is formed from the mantle.