MESOZOIC PLATE UNDERFLOW, COLLISION, CONTINENT FORMATION AND HYDROTHERMAL GOLD DEPOSITS ALONG THE NORTH PACIFIC RIM

Numerous gold vein deposits formed in the Dabie-Sulu belt of east-central China + central Korea + SW Japan + Sikhote Alin, and in the Klamath Mountains + Sierran Foothills of northern California attending ~155-90 Ma subduction of paleo-Pacific lithospheric plates. In eastern Asia, earlier transpression and continental collision at ~305-210 Ma generated a high pressure-ultrahigh pressure orogen, but failed to produce widespread intermediate and felsic calcalkaline, I-type magmatism or abundant hydrothermal gold deposits. Similarly in northern California, strike slip ± minor transtension-transpression over the interval ~380-160 Ma resulted in the episodic stranding of ophiolitic chert-argillite terranes, but generated few granitoid magmas or Au ore bodies. For both continental margin realms, periods of nearly head-on Late Jurassic-Cretaceous subduction of oceanic lithosphere involved long-sustained underflow; on reaching magmagenic depths of ~100 km, the descending mafic-ultramafic plates dewatered due to the breakdown of hornblende, chlorite and serpentine, producing voluminous calcalkaline, I-type arc magmas This process resulted in the abundant formation of sialic crust. Ascent of these plutons into the middle and upper crust released CO2 ± S-bearing hydrothermal fluids and/or devolatilized the contact-metamorphosed wall rocks. Such hot aqueous fluids transported gold along fractures and fault zones, precipitating it locally in response to cooling, fluid mixing, and/or reactions with wall rocks of contrasting compositions (e.g., serpentinite, marble). In contrast, where continental crust was subducted to depths of ~100 km or more, little devolatilization took place, reflecting the thermal stabilities of white mica and biotite. Accordingly, only minor production of S-type granitoid melts occurred, and few major coeval Au vein deposits formed. Thus, the generation of new quartzofeldspathic crust and mobilization of precious metal-bearing fluids in continental margin and island arc environments seems to require the long-continued, nearly orthogonal descent of oceanic—not continental—lithosphere. Continental crustal thickness in such active convergent margins is mainly a function of the rate of generation of juvenile, I-type intermediate and felsic magmas coupled with contractional or extensional deformation, but to a lesser extent depends upon precipitation-modulated erosion. The NS-trending Andes surmount the South American lithosphere above the east-descending Nazca plate. The highest mountains, underlain by ~70 km-thick crust, cap the extremely arid Andean Cordillera at ~25° S. Farther south, the precipitation-drenched Bernard O'Higgins fjordland at ~45° S is underlain by only ~35 km-thick crust. The Cascade + Sierra Nevada ranges in the western U. S. display comparable NS trends and latitudinal rainfall patterns. Westerly winds supply abundant moisture to the Pacific Northwest, but precipitation diminishes southward, producing increased aridity where the Sierra achieves its maximum regional elevation in the Kern Plateau surrounding Mount Whitney. In spite of abundant calcalkaline volcanism and plutonism, island arcs and continental margins rimming much of the North Pacific Basin, including the Japanese Islands, Kamchatka Peninsula, and the Alaska-Aleutians belts, are supported by I-type crust of only moderate thickness; these arcs and continental margins are drenched in rain and snowfall, hence the crustal thickness is at least in part due to the counteracting effect of surface erosion.