Role of crustal melting in petrogenesis of the Cretaceous Water Island Formation (Virgin Islands, northeast Antilles Island arc)

The latest Aptian to earliest Albian (~115 Ma) Water Island Fm in the Virgin Islands contains some of the oldest known arc-related strata in the Greater Antilles Island Arc. Hence, the unit is of considerable significance in tectonic reconstructions of initial subduction parameters along the long-lived destructive plate margin separating the North American and Caribbean Plates. Exposed Water Island strata are bimodal, consisting predominantly of altered dacite and rhyolite (originally called keratophyre; 65-85% SiO2) and subordinate degraded (spilite; 46-57% SiO2). TiO2 content of Water Island basalt averages approximately 0.5%, resembling borderline intermediate-Ti boninite basalts, consistent with low incompatible element abundances and low normalized light rare earth elements (LREE) with respect to Sm. Trace element patterns of the felsic suite, characterized by pronounced negative normalized anomalies for high field-strength elements (HFSE), low Sr/Y, and low absolute rare earth element (REE) abundances, and relatively flat normalized REE patterns, have analogues in plagiorhyolite suites from bimodal Cenozoic arcs, including the western Aleutians, Izu-Bonin, the Kermadecs, and South Sandwich. Relatively low incompatible element concentrations in plagiorhyolites and contrasting normalized incompatible trace element patterns in basalts preclude an origin of Water Island plagiorhyolite through MORB-type fractional crystallization. Compositions are consistent instead with melting models involving partial fusion of amphibole-bearing gabbro at low pressures (within the stability range of plagioclase) in response to introduction of heat and aqueous flux by arc-related basalt melts and associated hydrothermal fluids during transmission to the surface. Truncation of the basalt fractional crystallization trend at SiO2 = 57% indicates evolved island arc basalt (IAB) crystal fractionates were gradually displaced from crustal magma conduits by more buoyant plagiorhyolite melt, and trapped in underplated, sub-crustal magma chambers. Basalts have low (Ce/Ce*)N (average ˜ 0.78), indicating the presence of significant pelagic sediment (0.5 to 1.5% Atlantic Cretaceous pelagic sediment, AKPS). One subunit of relatively high-HFSE plagiorhyolite has (Ce/Ce*)N near-expected values, but another with low-HFSE has slightly lower than expected (Ce/Ce*)N, consistent with a small sediment component. Absence of intermediate andesite from the Water Island Fm is inconsistent, however, with basaltrhyolite magma mixing processes. Consequently, incorporation of sediment by low-HFSE plagiorhyolite is inferred to have resulted from re-melting of arc-related gabbro

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)

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)

Exploration for hot dry rock geothermal resources in the Midcontinent USA. Hot dry rock conceptual models for exploration, HDR test site investigations, and the Illinois Deep Drill Hole Project. Volume 2

Three potential sources of HDR, each covering approximately a 2/sup 0/ x 2/sup 0/ area, were identified and subjected to preliminary evaluation with ad hoc exploration strategies. In the Mississippi Embayment test site, lateral thermal conductivity variations and subcrustal heat sources may be involved in producing abnormally high subsurface temperatures. Studies indicate that enhanced temperatures are associated primarily with basement rift features where vertical displacement of aquifers and faults cause the upward migration of hot waters leading to anomalously high, local, upper crustal temperatures. The Western Nebraska test site is a potential low temperature HDR source also related, at least in part, to groundwater movement. There appear to be much more widespread possibilities for similar HDR sites in the Great Plains area. The Southeast Michigan test site was selected for study because of the possible presence of radiogenic plutons overlain by a thickened sedimentary blanket. There is no direct information on the presence of abnormally high temperatures in this area, but the study does show that a combination of gravity and magnetic anomaly mapping with regional geological information derived from sparse drill holes in the Phanerozoic rocks is useful on a widespread basis for focusing on local areas for detailed evaluation

Electromagnetic image of the Trans-Hudson orogen — THO94 transect

Author Posting. © National Research Council Canada, 2005. This article is posted here by permission of National Research Council Canada for personal use, not for redistribution. The definitive version was published in Canadian Journal of Earth Sciences 42 (2005): 479-493, doi:10.1139/E05-016.The North American Central Plains (NACP) anomaly in enhanced electric conductivity and its relationship with the Paleoproterozoic Trans-Hudson orogen (THO) has been studied under the auspices of Lithoprobe for over a decade. The NACP anomaly was the first geophysical evidence of the existence of the THO beneath the Phanerozoic sediments of the Central Plains. This anomaly, detected geomagnetically in the late 1960s, has been the subject of a number magnetotelluric studies from the early 1980s. The PanCanadian and Geological Survey of Canada experiments in the 1980s and the Lithoprobe experiments in the 1990s together comprise four east–west and one north–south regional-scale profiles in Saskatchewan perpendicular to the strike of the orogen. In this paper, data from the northernmost line, coincident with seismic line S2B, are analysed and interpreted, and are shown to be key in determining the northern extension of the NACP anomaly. Dimensionality analysis confirms the rotation of deep crustal structures eastward to Hudson Bay, as earlier proposed on the basis of broad-scale geomagnetic studies. On this profile, as with the profile at the edge of the Paleozoic sediments, the NACP anomaly is imaged as lying within the La Ronge domain, in contact with the Rottenstone domain, and structurally above the Guncoat thrust, a late compressional feature. The location of the anomaly together with the surface geology suggests that the anomaly is caused either by sulphide mineralization concentrated in the hinges of folds, by graphite, or by a combination of both. Our interpretation of the data is consistent with that from other profiles, and suggests that the NACP anomaly was formed as a consequence of subduction and collisional processes involving northward subduction of the internides of the THO beneath the Hearne craton. On the southern part of this profile, a resistive structure is identified as the Sask craton, suggesting that Proterozoic rocks are above Archean rocks in the THO.XG was supported by a fellowship of the Spanish Ministry of Science: PB92-0808