Both copper and zinc were mobilized during prehnite-pumpellyite facies metamorphism in the Keweenawan basalts of northern Michigan. Cu, averaging 70 ppm in basalts, was leached from epidotized or more highly metamorphosed rocks in the zone of dehydration; it was introduced into hydrated rocks where pumpellyite, prehnite, laumontite, and chlorite are the principal hydrous secondary phases. Native copper is a common associate of the metamorphic assemblage in such rocks. The chemical data suggest that fluids evolved by dehydration at depth may well have contained from 1,500 to 2,000 ppm Cu. These fluids percolated upward toward lower temperature levels where hydration was dominant and where the residual concentration of Cu-ions in the fluids could have reached high levels if precipitation had not occurred. The model supplies nearly five orders of magnitude more copper than has been produced by a century of mining in the Keweenaw Peninsula. The data suggest that enrichment or depletion of native copper should be spatially related to the distribution of certain hydrous minerals in the Keweenawan lava pile. Zn, which averages 100 ppm in unaltered olivine tholeiites, was leached from all highly altered rocks in permeable zones and removed by metamorphic fluids. Abundance of Ni was not appreciably affected by the metamorphic process except where there was dilution by void-filling secondary minerals. Introduction TI-XE Keweenawan tholeiitic lavas of northern Michi-gan, called collectively the Portage Lake Volcanics, underwent a postdepositional metamorphic event which produced secondary mineral assemblages of the zeolite and prehnite-pumpellyite facies (Stoiber and Davidson, 1959; Jolly and Smith, 1972). In the upper part of the five-kilometer-thick section exposed in the Keweenaw Peninsula, albite, prehnite, laumontite, chlorite, minor analcime, and sphene, along with rare native copper, are present in metamorphosed rocks. In the middle part of the section, albite, pumpellyite, prehnite, native copper, quartz, chlorite, sphene, and minor epidote are characteristic of altered lavas. Finally, in the lowest exposed rocks, albite, epidote, quartz, chlorite, sphene, and minor pumpellyite form metamorphic assemblages. These three parts of the lava pile will here be called the chlorite, pumpellyite, and epidote zones respectively after Jolly and Smith (1972). The purpose of this report is to suggest relationships between the metamorphic process and behavior of three common trace metals, Cu, Zn, and Ni. Metamorphic PetrologyBecause the trace element redistribution is intimately related to the metamorphic process, the' major characteristics of this alteration must be briefly summarized. More complete information on this topic may be found in Stoiber and Davidson (1959), White (1968), and Jolly and Smith (1972). The metamorphic alterations of the presently exposed Keweenawan lavas were localized in permeable parts of the lava pile only; extensive readjustment took place primarily in amygdular zones, breccias, conglomerate units, o...

Island arc volcanic strata in Puerto Rico, ranging in age from Lower Cretaceous (Aptian) to Eocene and dating from about 120 to 45 Ma, represent one of the longest oceanic arc sequences preserved in the world. Detailed and systematic mapping by the U.S. Geological Survey published between 1959 and 1986 reveal that, although postvolcanic sedimentary platform deposits consisting of limestone and other detrital materials ring the island and cover extensive parts of the arc platform, representative strata of the entire sequence are exposed. The island is subdivided into three volcanic provinces all containing strata without correlative units elsewhere; they are (1) a northeastern volcanic province, separated from the central province by the Cerro Mula Fault, a left-lateral strike-slip fault of mid-Santonian age with displacement of at least 50 km; (2) a central volcanic province, dominated by volcanic debris accumulated during sequential development of five east-west-oriented volcanic belts, and (3) a western volcanic province, with a northwest-southeast-trending boundary of uncertain origin, containing remnants of two sequential island arc volcanic belts of CampanianMaastrichtian and Eocene age.Additionally, in the southwestern corner of the island, the Sierra Bermeja Complex consists of a tectonic melange of partly serpentinized ultramafic rocks representing the lithospheric upper mantle originally composed of spinel-bearing peridotites, including lherzolite, harzburgite, and dunite. The melange incorporates rafts, blocks, and boulder-sized clasts of (1) Lower Jurassic to Upper Cretaceous pelagic sediments (Mariquita Chert), including radiolarian chert of Pacific provenance and later siliceous volcanogenic strata, representing pre-island arc oceanic deposits; and (2) altered mid-ocean ridge basalt (MORB)-like basalts (Cajul Basalt) and amphibolites (Las Palmas amphibolite melange) of probable Lower Jurassic age, representing pre-island arc oceanic crust. Serpentinized peridotite bodies, containing amphibolite, altered MORB-like basalt, and chert, protruded the arc platform in a cold state at several places in the western province. The largest of these, the Monte del Estado Peridotite, was emplaced during the Maastrichtian, through crustal extension associated with left-lateral strike-slip faulting along the Cordillera fault.Volcanic strata preserved in the central province are subdivided into five major volcanic phases on the basis of stratigraphic and geochemical relations. Basalts evolved progressively from early primitive island arc tholeiites in phase I, to calcalkaline basalts in phase II, and finally to incompatible element-enriched shoshonite Jolly, W. T.

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Eastern Puerto Rico is composed of two distinctive volcanic terranes, the northeastern and central volcanic provinces, that represent independent tectonic blocks assembled in mid-Santonian time by left-lateral strike-slip faulting with displacement of at least 50 km. Volcanic strata in the juxtaposed tectonic blocks occur in five successive volcanic phases: (I) Upper Aptian to Lower Albian island arc tholeiites (IAT); (II) Upper Albian cal-calkaline (CA) volcanic rocks; (III) Cenomanian to Lower Santonian shoshonites (SHO, central block) and cal-calkaline (northeastern block) volcanic rocks; (IV) Upper Santonian to Maastrichtian cal-calkaline to shoshonitic volcanic rocks; and (V) Lower Tertiary, predominantly felsic, cal-calkaline volcanic rocks. The geochemical evolution is marked by progressive increases in ratios of large-ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength (HFSE) and heavy rare earth (HREE) elements. Compositional changes with time are also evident in the Nd isotope ratios of the volcanics of the central tectonic block, which are MORB-like and display a gradual decrease in ε Nd from +8 to about +6 between phases I and IV. In contrast, Sr and Pb isotope ratios show no evidence of compositional evolution. Initial (i) Sr isotope compositions overlap the field of Mesozoic MORB with i 87 Sr/ 86 Sr ratios exhibiting a broad range from about 0.7034 to 0.7044 (ε Sr from -12 to +1.0) and an average of 0.7037 (-10.0). Pb isotope compositions form a trend emanating from the MORB field and oriented along or subparallel to the Northern Hemisphere Reference Line (NHRL).HFSE ratios in Puerto Rican basalts overlap the MORB trend at all concentrations, consistent with incompatible behavior. As in modern island arcs, low absolute HFSE abundances compared with MORB indicate the wedge was more depleted in incompatible elements than a MORB-type source. Uniformly flat normalized HREE patterns, together with wide ranges in Nb/Hf ratios reflecting variation in degree of melting, indicate magma generation at relatively low P t . Cooling of the shallow wedge during the 35-Ma eruption period is inferred to have produced the observed geochemical shifts. Superimposed on the magmas are effects of low-P t gabbroic and, in