Pre-arc basement complex and overlying early island arc strata, Southwestern Puerto Rico : overview, geologic evolution, and revised data bases

The pre-arc basement complex in southwestern Puerto Rico consists of rocks exposed in the Bermeja complex. The oldest rocks are highly serpentinized peridotites that occur in three belts (Monte del Estado, Río Guanajibo, and Sierra Bermeja). These serpentinites were emplaced into a sequence of Jurassic to mid-Cretaceous pelagic chert (Mariquita chert) that contains abundant rafts and blocks of N-MORB-type amphibolites (Las Palmas amphibolite) and tholeiite and associated trondhjemite fractionates (Lower Cajul MORB) also of N-MORB affinity. The rocks are apparently overlain by a younger sequence of pre-arc plateau basaltic and andesitic lava flows (Upper Cajul Formation) that occur in two distinct geographic sequences, one having E-MORB and the other OIB geochemical characteristics. Overlying these pre-arc rocks in western Puerto Rico are northwest-trending Late Cretaceous to Eocene (85 to 45Ma) island arc strata that chronologically overlap later volcanic phases in central Puerto Rico. These western Puerto Rico arc rocks have elevated incompatible element concentrations together with conspicuously shallow negative Nb-anomalies, slightly positive Zr-Hf anomalies, and exceedingly high OIB-like Nb/Zr, all indicative of enriched source compositions. Trace element patterns are reproduced by multiple component mixing models involving highly depleted spinel peridotite (RMM15 to 20) overprinted by small OIB-type (up to ~2%) and pelagic sediment components. Trace element abundances are too high to qualify Atlantic Cretaceous pelagic sediment as a potential contaminant, but mantle-melting models (f=0.25) are consistent with the incorporation of variable proportions of Caribbean Cretaceous pelagic sediment through northdipping subduction of the Caribbean basin. Anomalous two-pyroxene-bearing andesites with extraordinarily high SiO2/MgO compared with normal mantle basaltic compositions, also indicate the incorporation of Jurassic to Early Cretaceous pelagic chert from the Caribbean. The high degree of source enrichment in western Puerto Rico is inconsistent with regional within-plate plume tectonics. Instead, it is inferred that the younger north-dipping western Puerto Rico arc (dating from ~85Ma) sampled an upper mantle enrichment zone generated in the backarc region of the older (125 to ~85Ma) south-dipping arc system in central Puerto Rico

Cretaceous to Mid-Eocene pelagic sediment budget in Puerto Rico and the Virgin Islands (northeast Antilles Island arc)

Island arc basalts (IAB) in the Greater Antilles, dating between Albian and mid-Eocene time (~112 to 45 Ma), consist of an early low-K, primitive island arc (PIA) basalt series and a later, predominantly intermediate calcalkaline (CA) series. The rocks resemble modern sediment-poor, low-light rare earth element (LREE)/heavy rare earth element (HREE) arc basalts from intra-oceanic tectonic settings and sediment-rich, high-LREE/HREE types from continental margin arcs, respectively. Isotope and incompatible trace element distribution along a 450 km segment of the arc in the northeast Antilles demonstrates that low-LREE/HREE basalts predominate in Albian to Santonian (~85 Ma) stratigraphic sequences in the Virgin Islands (VI) and northeast Puerto Rico (NEPR), while there is a gradual but spectacular increase in both LREE/HREE and absolute abundances of incompatible elements in central Puerto Rico (CPR). Northeastern Antilles basalts have consistently elevated La/Nb and relatively low Nb/Zr, both inconsistent with the presence of a significant ocean island basalt component. Hence, observed differences are interpreted to reflect variation in proportions of pelagic sediment subducted by the south-dipping Antilles arc system as it swept north-eastward across the Caribbean region and eventually approached the Bahama Banks along the south-eastern fringes of the North American Plate. Trace element mixing models indicate sediment proportions in VI and NEPR were limited, averaging considerably below 1.0%. In comparison sediment content in CPR increased from an average slightly above 1.0% in Albian (~112 Ma) basalts to as high as 8% in Cenomanian (100-94 Ma) types. Hypothetical pre-arc pelagic sedimentary facies in the subducted proto-Atlantic (or proto-Caribbean) basin, included 1) a young, centrally located longitudinal ridge-crest facies, with a thin sediment cover, eventually subducted by VI and NEPR, 2) a slightly older basin-margin facies of variable width and moderate sediment thickness, subducted by CPR during Albian time, and 3) a thick, pre-arc continental margin facies in the vicinity of Central America, subducted by CPR during Cenomanian time. Following collision of neighboring Hispaniola with the Bahamas sediment budgets in the northeast Antilles stabilized at moderate levels from 2 to 3%, reflecting widespread subduction of North Atlantic Cretaceous pelagic sediment (AKPS)

Sm/Nd evidence for a major 1.5 Ga crust-forming event in the central Grenville province

The majority of gray gneisses from an 80 000 km2 area in the central (Quebec) Grenville province define a Sm-Nd isochron that yields an age of 1.53 ±0.07 Ga. Major element, trace element, and initial Nd isotopic data all point to a subduction-related (orogenic) source for this terrane. It is attributed to a major crustal extraction event that formed a mid-Proterozoic island arc. The age of the terrane agrees with published data from the eastern and western Grenville province in Newfoundland and Ontario, respectively. We suggest that 1.5 Ga are fragments were accreted along much of the length of the Laurentian foreland within 100 m.y. of their formation, in an event marked by widespread granitoid plutonism

Early Proterozoic Melt Generation Processes beneath the Intra-cratonic Cuddapah Basin, Southern India

Early Proterozoic tholeiitic lavas and sills were emplaced during the initial phase of extension of the intra-cratonic Cuddapah Basin, southern India. Ar-40-Ar-39 laser-fusion determinations on phlogopite mica,from the Tadpatri Fm mafic-ultramafic sill complex, constrain the age of the initial phase of extension and volcanism in the basin at 1.9 Ga. Despite their Early Proterozoic age, the igneous rocks are unmetamorphosed, undeformed and remarkably fresh. They exhibit a wide range in MgO contents (4-28 wt %) and have undergone varying degrees of accumulation or crystal fractionation. Variable La/Nb ratios (1.2-3.7) and epsilon(Nd) values (1 to -10) suggest that some, but not all, of the mafic rocks have been affected by crustal contamination. This appears to have taken place in magma chambers at similar to9 kbar, i.e. the base of the continental crust. Forward modelling of major and trace elements (Fe and Nd) and inverse modelling of rare earth elements suggest that the primary Cuddapah melts were generated by similar to10-15% partial melting of a lherzolite mantle source. This corresponds to a mantle Potential temperature of similar to1500degreesC. The thickness of the mechanical boundary layer predicted by the geochemical modelling is 70 km with a minimum initial lithospheric thickness of 120 kin. This corresponds to a stretching factor of 1.6-1.8. Richter's (1988) secular cooling model for the Earth predicts that, at 1.9 Ga, the ambient mantle had a potential temperature of similar to1500degreesC (i.e. similar to200degreesC hotter than Phanerozoic mantle). If the cooling model is correct then Proterozoic lithospheric stretching and mantle melting beneath the intra-cratonic Cuddapah Basin could have been caused by passive rather than active rifting

Pre-arc basement complex and overlying early island arc strata, Southwestern Puerto Rico : overview, geologic evolution, and revised data bases

The pre-arc basement complex in southwestern Puerto Rico consists of rocks exposed in the Bermeja complex. The oldest rocks are highly serpentinized peridotites that occur in three belts (Monte del Estado, Río Guanajibo, and Sierra Bermeja). These serpentinites were emplaced into a sequence of Jurassic to mid-Cretaceous pelagic chert (Mariquita chert) that contains abundant rafts and blocks of N-MORB-type amphibolites (Las Palmas amphibolite) and tholeiite and associated trondhjemite fractionates (Lower Cajul MORB) also of N-MORB affinity. The rocks are apparently overlain by a younger sequence of pre-arc plateau basaltic and andesitic lava flows (Upper Cajul Formation) that occur in two distinct geographic sequences, one having E-MORB and the other OIB geochemical characteristics. Overlying these pre-arc rocks in western Puerto Rico are northwest-trending Late Cretaceous to Eocene (85 to 45Ma) island arc strata that chronologically overlap later volcanic phases in central Puerto Rico. These western Puerto Rico arc rocks have elevated incompatible element concentrations together with conspicuously shallow negative Nb-anomalies, slightly positive Zr-Hf anomalies, and exceedingly high OIB-like Nb/Zr, all indicative of enriched source compositions. Trace element patterns are reproduced by multiple component mixing models involving highly depleted spinel peridotite (RMM15 to 20) overprinted by small OIB-type (up to ~2%) and pelagic sediment components. Trace element abundances are too high to qualify Atlantic Cretaceous pelagic sediment as a potential contaminant, but mantle-melting models (f=0.25) are consistent with the incorporation of variable proportions of Caribbean Cretaceous pelagic sediment through northdipping subduction of the Caribbean basin. Anomalous two-pyroxene-bearing andesites with extraordinarily high SiO2/MgO compared with normal mantle basaltic compositions, also indicate the incorporation of Jurassic to Early Cretaceous pelagic chert from the Caribbean. The high degree of source enrichment in western Puerto Rico is inconsistent with regional within-plate plume tectonics. Instead, it is inferred that the younger north-dipping western Puerto Rico arc (dating from ~85Ma) sampled an upper mantle enrichment zone generated in the backarc region of the older (125 to ~85Ma) south-dipping arc system in central Puerto Rico

Cretaceous to Mid-Eocene pelagic sediment budget in Puerto Rico and the Virgin Islands (northeast Antilles Island arc)

Island arc basalts (IAB) in the Greater Antilles, dating between Albian and mid-Eocene time (~112 to 45 Ma), consist of an early low-K, primitive island arc (PIA) basalt series and a later, predominantly intermediate calcalkaline (CA) series. The rocks resemble modern sediment-poor, low-light rare earth element (LREE)/heavy rare earth element (HREE) arc basalts from intra-oceanic tectonic settings and sediment-rich, high-LREE/HREE types from continental margin arcs, respectively. Isotope and incompatible trace element distribution along a 450 km segment of the arc in the northeast Antilles demonstrates that low-LREE/HREE basalts predominate in Albian to Santonian (~85 Ma) stratigraphic sequences in the Virgin Islands (VI) and northeast Puerto Rico (NEPR), while there is a gradual but spectacular increase in both LREE/HREE and absolute abundances of incompatible elements in central Puerto Rico (CPR). Northeastern Antilles basalts have consistently elevated La/Nb and relatively low Nb/Zr, both inconsistent with the presence of a significant ocean island basalt component. Hence, observed differences are interpreted to reflect variation in proportions of pelagic sediment subducted by the south-dipping Antilles arc system as it swept north-eastward across the Caribbean region and eventually approached the Bahama Banks along the south-eastern fringes of the North American Plate. Trace element mixing models indicate sediment proportions in VI and NEPR were limited, averaging considerably below 1.0%. In comparison sediment content in CPR increased from an average slightly above 1.0% in Albian (~112 Ma) basalts to as high as 8% in Cenomanian (100-94 Ma) types. Hypothetical pre-arc pelagic sedimentary facies in the subducted proto-Atlantic (or proto-Caribbean) basin, included 1) a young, centrally located longitudinal ridge-crest facies, with a thin sediment cover, eventually subducted by VI and NEPR, 2) a slightly older basin-margin facies of variable width and moderate sediment thickness, subducted by CPR during Albian time, and 3) a thick, pre-arc continental margin facies in the vicinity of Central America, subducted by CPR during Cenomanian time. Following collision of neighboring Hispaniola with the Bahamas sediment budgets in the northeast Antilles stabilized at moderate levels from 2 to 3%, reflecting widespread subduction of North Atlantic Cretaceous pelagic sediment (AKPS)

Early Cretaceous Basalt and Picrite Dykes of the Southern Etendeka Region, NW Namibia: Windows into the Role of the Tristan Mantle Plume in Paraná–Etendeka Magmatism

Abundant dykes in the southern Etendeka region, NW Namibia, mostly contain 8–20% MgO. Almost all can be allocated to previously described Early Cretaceous magma types. Horingbaai-type basalts–picrites occur up to 120 km inland. Some have superficially mid-ocean ridge basalt (MORB)-like compositions: (La/Nb)n 1·0; (Sm/Lu)n 1·5; Nd >8; initial 87Sr/86Sr 0·7032. Others show major- and trace-element, and Sr–Nd–Pb isotopic evidence of contamination during upwelling by up to a few per cent of K-feldspar-rich upper crust. Extremely magnesian olivine macrocrysts (Fo91–93·3) in some Horingbaai picrites indicate that komatiitic (MgO 24%) liquids were associated with this suite, although they were too dense to reach the crustal levels currently exposed. Ferropicrite dykes resemble closely the nearby Etendeka ferropicrite basal lavas, with total iron (as Fe2O3) >MgO; (La/Nb)n 1·0–1·5; (Sm/Lu)n 2·5–7; Nd 2–3, except in samples with geochemical evidence of contamination by Archaean–Proterozoic lower crust. A few low-Ti dykes resemble geochemically the lavas of the main southern Etendeka succession, with (La/Nb)n 2·0–2·5; Nd = 0 to -8; initial 87Sr/86Sr >0·707. Dykes north of the Huab river define a fourth magma type, Nil Desperandum (ND), that may have fed part of the Huab sill complex. The dyke ages are constrained by their field relationships. Ferropicrite and low-Ti dykes are consanguineous with lavas erupted at 133 and 132 Ma, respectively. Both Horingbaai and ND dykes cut 132 Ma Etendeka lavas, and the main swarm of Horingbaai picrites with forsteritic macrocrysts is cut by the 131 Ma Brandberg plutonic complex. Forward and inverse modelling of the genesis of the ferropicrites and the Horingbaai picrite–komatiites gives two temporal ‘windows’ into physicochemical conditions within the head of the impacting Tristan mantle plume. At 133 Ma the southern Etendeka lithosphere was >100 km thick and only incipient melting of predominantly Fe-rich peridotite streaks occurred in the rising plume head (Tp = 1500°C). By 2 my later, pre-Atlantic extension had reduced the lithosphere thickness beneath and adjacent to NW Namibia, and there was intense melting (Tp = 1500–1700°C) of even depleted peridotite in the plume head

Ankerite carbonatite from Swartbooisdrif, Namibia: the first evidence for magmatic ferrocarbonatite

Although general accounts of carbonatites usually envisage Ca-Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe2+)(CO3)2 cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe3O4. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420-460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (87Sr/86Sr=0.7033; )Nd=0.2-1.0; 206Pb/204Pb=16.37; 207Pb/204Pb=15.42; 208Pb/204Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate '18O=8.0 and '13C=-7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock