Wayne T. Jolly

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).

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.

Petrogenesis of fertile mantle peridotites from the Monte del Estado massif (southwest Puerto Rico): a preserved section of Proto-Caribbean lithospheric mantle?

The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal mantle rocks from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to alteration processes but probably reflects the capture of percolating fluid/melt fractions or the crystallization of sub-percent amounts of hydrous minerals (e.g., amphibole, phlogopite) along grain boundaries or as microinclusions in minerals. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic- Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processes

Generation of Archean tonalites

Thermal evolution models of young subducting oceanic plates under Archean conditions suggest that tholeiitic basalts in the subducting wet crust melted at shallow depths (50 to 80 km) and produced Archean tonalites. The latent heat of fusion consumed by the crust kept the overlying peridotitic mantle wedge below the melting temperature of peridotite. This prevented both fusion in the wedge and production of cale-alkaline island-are volcanic rocks similar to those observed in modern subduction zones.

The case for persistent southwest-dipping Cretaceous convergence in the northeast Antilles: Geochemistry, melting models, and tectonic implications

Constraints on the polarity of Cretaceous subduction in the Greater Antilles are provided through geochemical comparison between the erupted island arc lavas in central Puerto Rico and potential pelagic sediment reservoirs in the fl anking ocean basins. Early Jurassic to mid-Cretaceous (185- to 65-Ma) sediment from the open Pacifi c on the southwest is dominated by pelagic chert, which is highly refractory and depleted with respect to incompatible elements. In comparison, mid- to Late Cretaceous (ca. 112- to 65-Ma) sediment from the younger Atlantic basin on the northeast was dominated by mixtures of two end members. These include (1) biogenic clay and carbonates with elevated light rareearth element (LREE) abundances, negative MORB-normalized, high fi eld-strength element (HFSE) anomalies, and low Zr/Sm; and (2) turbiditic detritus of upper continental crust composition with high LREE, comparatively shallow HFSE anomalies, and high Zr/Sm. Compositions of Puerto Rican arc basalts are inconsistent with incorporation of Pacifi c pelagic chert. Instead, patterns characteristic of high-Fe island arc tholeiites are reproduced by incorporation of up to 4% of a low-Zr/Sm biogenic sediment component of Atlantic origin, whereas patterns of low-Fe lavas require, in addition to biogenic sediment, introduction of up to 2% of a high-Zr/ Sm crustal turbidite component. The Atlantic origin of all the subducted sediments indicates the polarity of subduction throughout the Cretaceous in the northeast Antilles was persistently southwest dipping. This conclusion is supported by the presence of a lowZr/Sm suprasubduction zone component of Atlantic origin in Caribbean plateau basalts (91‐88 Ma) from southwest Puerto Rico, which were erupted within the broad backarc region of the Greater Antilles during intermediate stages of arc development.

Subdivision of the Archean lavas of the Abitibi area, Canada, from Fe-Mg-Ni-Cr relations

Abstract The lavas of a part of the Archean Abitibi region may be divided into three stratigraphic levels in each of which FeO-MgO-Ni-Cr contents conform to certain broad differentiation trends. Within each stratigraphic level, there is a tendency for rocks to become more felsic upwards. The earliest and stratigraphically lowest subdivision is composed largely of magnesium-rich basaltic lavas called the magnesian suite. In the central part of the pile, where basalts predominate, the lavas contain intermediate MgO concentrations, and display pronounced Fe enrichment in intermediate members similar to conventional tholeiites. In the latest and stratigraphically highest lavas, where andesites predominate, Fe depletion is characteristic; these lavas are grouped into a primitive calcalkaline suite. All of the Abitibi lavas contain unusually high Ni and Cr. Other Archean lava piles appear to be similarly divisible, although all three suites are not always present. Mafic end-members of the three complete differentiation suites are viewed as possible source magmas derived by partial melting in a primitive, olivine-rich parent, probably the Archean mantle. The earliest, and highest temperature magmas precipitated olivine, Al-clinopyroxene, and minor Al-orthopyroxene, and display moderate FeO, TiO 2 , MnO, Al 2 O 3 , and CaO enrichment in more felsic members. The intermediate age lavas, derived originally by less complete melting in the parent, precipitated plagioclase, olivine, and lesser clinopyroxene, and display, as a result, strong Fe enrichment until, in intermediate members, magma volumes became small enough to yield P f of levels sufficient to form clinopyroxene plus magnetite. The uppermost lavas, derived by relatively small volumetric melting in the parent, contain abundant Fe-Ti oxides in even the most mafic members, along with augite and plagioclase.

Geochemical stratigraphy of the Huronian continental volcanics at Thessalon, Ontario: contributions of two-stage crustal fusion

The 1500 m thick sequence of Huronian continental volcanics at Thessalon, Ontario is subdivided into 4 volcanic cycles, each of which includes abundant early mafic end-members, central intermediate flows, and late rhyolite units. Major and trace element concentrations are dominated by extensive gabbroic fractionation trends that ultimately produced two types of felsic flows: (1) rhyolites with high light rare earth element (LREE) and relatively low large-ion lithophile element (LILE) concentrations (high-LREE, low-LILE rhyolites), and (2) following late separation of REE-rich accessory phases, rhyolites depleted in LREE (low-LREE, high-LILE rhyolites). Mafic end-members of individual volcanic cycle are progressively less siliceous and less enriched in LILE and LREE with height in the stratigraphic section. Ti/Zr ratios gradually rise from 35 in early mafic flows to stabilize at about 85 in late units, while average SiO2 contents decrease from 56 to about 50% and Mg# rises from about 48 to 52. ɛ-Nd values are consistently negative, indicating variable degrees of pre-fractionation crustal contamination of the end-member magmas during their uprise through the crust. Mixing models are consistent with up to 50% contamination by crustal material of tonalitic hornblende-gneiss composition. A progressive increase in ɛ-Nd, from about-5.0 to-0.5 upward in the volcanic succession, reflects a decreasing degree of crustal contamination due to development of insulating layers along margins of the feeder system. Detailed stratigraphic variations suggest that successive magmas batches were intercepted by a progressively fractionating, periodically replenished magma source, giving rise to open-system magmatism. Despite the prevalence of crustal assimilation in the Huronian lavas, (La/Sr)N ratios are too low in least contaminated end-members to be explained by contamination of tholeiitic magmas. The late basalts resemble instead modern island are basalts, and it is suggested that the subcontinental mantle source was enriched by subduction-related processes during crustal formation. Within individual volcanic cycles gabbroic fractionation trends systematically deviate from calculated factors toward compositions characteristic of hornblende-gneiss. Such relations suggest that further crustal contamination of the magmas occurred simultaneous with crystal fractionation. probably within undulating sills at upper crustal levels. Quantitative analysis suggests assimilation/fractional crystallization (A/FC) ratios of about 0.45. As a result of extensive two-stage contamination, rhyolites from the initial volcanic cycle incorporate a total of over 60% of crust.

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, Rio 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

Role of crustal contamination in heterogeneous Archean volcanics from the Leonora region, Western Australia

Abstract Archean supracrustal volcanic successions in the Leonora-Laverton region of the Eastern Goldfields Province, Western Australia, are subdivided into two stratigraphic associations: (1) an early succession ten km thick containing homogeneous high-Ti tholeiite basalts displaying nearly flat chondrite-normalized incompatible lithophile element patterns with (La/Sm)N ratios ranging from 0.88 to 1.12, and (2) an unconformably overlying heterogeneous succession four km thick consisting of (a) early siliceous, low-Ti tholeiites and associated intermediate calc-alkaline volcanics, grading to (b) high-Ti tholeiite similar to those of association (1), and (c) late siliceous high-Mg, spinifex-textured basalts. Fractionation models and trace element distribution in high-Ti basalts suggest that all the flows were derived from similar but not identical parental magmas, and that subvolcanic gabbroic fractionation was characteristic. Basaltic end-members of the siliceous, low-Ti suite had identical fractionation patterns, but hornblende joined the assemblage in contaminated intermediate calc-alkaline members, producing low-HREE fractionates. The siliceous basalts exhibit variable enrichment in large-ion lithophile (LILE) and light rate earth (LREE) elements with a continuum of (La/Sm)N ratios up to 2.75. They also have marked negative anomalies for Nb, P, and Ti; several samples also exhibit a positive anomaly for Zr on normalized distribution diagrams. Low Sr concentrations are inconsistent with mantle enrichment processes, and compositions of siliceous volcanics are explained instead by up to 30% contamination of high-Ti tholeiite liquids by Archean crust composed of hornblende tonalite gneiss. Geochemistry of the volcancis is consistent with eruption in a continental back arc basin tectonic setting.

Rare Earth Elements in the Geological Sciences

Rare earth elements (REE) are an integral part of modern geochemical and petrological studies. The rare earths are widely used, either alone as a group or in combination with other trace elements, to study igneous rock systems and, to a lesser extent, sedimentary and metamorphic rocks. Most of the major advances have been in the field of igneous petrogenesis and geochemistry, where they have been particularly important in evaluating the composition and history of magmatic source regions, the conditions of melting, the extent of melting, the modification of melt composition by assimilation or metasomatism, and the magmatic differentiation processes of the resulting magmatic system.