Michael R. Perfit

143Nd/144Nd,87Sr/86Sr and trace element constraints on the petrogenesis of Aleutian island arc magmas

Abstract 143 Nd/ 144 Nd, 87 Sr/ 86 Sr and trace element results are reported for volcanic and plutonic rocks of the Aleutian island arc. The Nd and Sr isotopic compositions plot within the mantle array with e Nd values of from 6.5 to 9.1 and 87 Sr/ 86 Sr ratios of from 0.70289 to 0.70342. Basalts have mildly enriched light REE abundances but essentially unfractionated heavy REE abundances, while andesites exhibit a greater degree of light to heavy REE fractionation. Both the basalts and andesites have significant large ion lithophile element to light rare earth element (LILE/LREE) enrichments. Variations in the isotopic compositions of Nd and Sr are not related to the spatial distribution of volcanoes in the arc, nor are they related to temporal differences. e Nd and 87 Sr/ 86 Sr do not correlate with major element compositions but do, however, correlate with certain LILE/LREE ratios (e.g. Ba N /La N ). Plutonic rocks have isotropic and trace element characteristics identical to some of the volcanic rocks. Rocks that make up the tholeiitic, calc-alkaline and alkaline series in the Aleutians do not come from isotopically distinct sources, but do exhibit some differing LILE characteristics. Given these elemental and isotopic constraints it is shown that the Aleutian arc magmas could not have been derived directly from homogeneous MORB-type mantle, or fresh or altered MORB subducted beneath the arc. Mixtures of partially altered MORB with deep-sea sediment can in principle account for the isotopic characteristics and most of the observed LILE/LREE enrichments. However, some samples have exceedingly high LILE/LREE enrichments which cannot be accounted for by sediment contamination alone. For these samples a more complex scenario is considered whereby dehydration and partial melting of the subducted slab, containing less than 8% sediment, produces a LILE-enriched (relative to REE) metasomatic fluid which interacts with the overlying depleted mantle wedge. The isotopic and LILE characteristics of the mantle are extremely sensitive to metasomatism by small percentages of added fluid, whereas major elements are not substantially effected, Major element compositions of Aleutian magmas are dominantly controlled by the partial melting of this mantle and subsequent crystal fractionation; whereas isotopic and LILE characteristics are determined by localized mantle heterogeneities.

Trace element and isotopic variations in a zoned pluton and associated volcanic rocks, Unalaska Island, Alaska: A model for fractionation in the Aleutian calcalkaline suite

Trace elements, including rare earth elements (REE), exhibit systematic variations in plutonic rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz monzodiorite and granodiorite. The chemical variations parallel those in the associated Aleutian calcalkaline volcanic suite. Concentrations of Rb, Y, Zr and Ba increase as Sr and Ti decrease with progressive differentiation. Intermediate plutonic rocks are slightly enriched in light REE (La/Yb=3.45–9.22), and show increasing light REE fractionation and negative Eu anomalies (Eu/Eu*=1.03–0.584). Two border-zone gabbros have similar REE patterns but are relatively depleted in total REE and have positive Eu anomalies; indicative of their cumulate nature. Initial 87Sr/86Sr ratios in 8 samples (0.70299 to 0.70377) are comparable to those of volcanic rocks throughout the arc and suggest a mantle source for the magmas. Oxygen isotopic ratios indicate that many of the intermediate plutonic rocks have undergone oxygen isotopic exchange with large volumes of meteoric water during the late stages of crystallization; however no trace element or Sr isotopic alteration is evident.Major and trace element variations are consistent with a model of inward fractional crystallization of a parental high-alumina basaltic magma at low pressures (〈6 kb). Least-squares approximations and trace element fractionation calculations suggest that differentiation in the plutonic suite was initially controlled by the removal of calcic plagioclase, lesser pyroxene, olivine and Fe-Ti oxides but that with increasing differentiation and water fugacity the removal of sub-equal amounts of sodic plagioclase and hornblende with lesser Fe-Ti oxides effectively drove residual liquids toward dacitic compositions. Major and trace element compositions of aplites which intrude the pluton are not adequately explained by fractional crystallization. They may represent partial melts derived from the island arc crust. Similarities in Sr isotopes, chemical compositions and differentiation trends between the plutonic series and some Aleutian volcanic suites indicates that shallow-level fractional crystallization is a viable mechanism for generating the Aleutian calcalkaline rock series.

Stratigraphic and Petrochemical Data Support Subduction Polarity Reversal of the Cretaceous Caribbean Island Arc

Stratigraphic and petrochemical data from island arc rocks in the NE Caribbean support previously proposed tectonic models of attempted subduction of the Caribbean oceanic plateau beneath the Caribbean island arc during pre-Aptian time (~119 Ma) and consequent reversal of arc polarity from SW-facing to NE-facing by the early Late Cretaceous (~97 Ma). We propose that reversal in subduction polarity is marked by an abrupt change from relatively depleted and non-radiogenic tholeiites of the primitive island arc series to more enriched and radiogenic volcanic rocks of the calc-alkaline series. The stratigraphic contact between the primitive island arc series and the overlying calc-alkaline series in central and eastern Hispaniola (Dominican Republic) and Puerto Rico is a major unconformity overlain by a shallow-marine limestone of Aptian-Albian age. We compare the characteristics of the proposed pre-Aptian Caribbean subduction reversal event to the better-documented Neogene subduction reversal event in the Solomon island arc in the SW Pacific.

Chemistry, origin and tectonic significance of metamorphic rocks from the Puerto Rico Trench☆

Abstract Constraints on the evolution of the northeastern Caribbean plate boundary and the tectonic development of the Puerto Rico Trench have been derived from a study of rocks and sediments dredged from thirty-six localities within and surrounding the trench. Marble, calc-schist, mica-schist, greenschist, amphibolite, magnesian schist and serpentinite crop out below the trench—slope break (∼ 3500 m) along the steep inner-trench wall for over 400 km. At shallower depths, limestone, calcilithite, and minor amounts of chert and sedimentary rock were sampled. Potassium—argon ages of two of the metamorphic rocks are 63 ± 3 and 66 ± 5 m.y.B.P. Rocks dredged from Mona Canyon, directly behind the inner-trench wall, are primarily igneous rocks, volcanic breccia, limestone and calcilithite that have been only slightly metamorphosed. Middle Eocene to Miocene (or younger) shallow-water limestones are interbedded and overlie the volcanogenic rocks. The outer-trench wall (oceanic side) is composed of tholeiitic basalts, serpentinite and deep-sea sediments commonly recovered from the Atlantic oceanic crust. Mineral assemblages and phase relations in the metamorphic rocks are similar to those observed in many subaerially exposed melanges believed to be subduction complexes. The conditions of metamorphism within the Puerto Rico Trench metamorphic complex are estimated to have been between 400 and 550°C and 3 to 7 kbar. Chemical analyses of the dredged samples suggest that: 1. (1) The metamorphic rocks from the inner-trench wall were primarily island-arc igneous rocks or sediments derived from them. 2. (2) Protoliths of the marbles and calc-schists were apparently biogenic carbonates and pelagic sediments with an arc-derived sedimentary component. 3. (3) Basalt and serpentinite from the outer-trench wall are geochemically similar to oceanic tholeiites (MORB) and ultramafics. 4. (4) Only two greenschists from the inner-trench wall have geochemical characteristics similar to MORB. 5. (5) Magnesian schists and serpentinite from deep along the inner wall have either been severely affected by metasomatism or represent metamorphosed ultramafic rocks from beneath the Greater Antilles. Our results indicate that, unlike a number of arc-trench systems in the Pacific, a great deal of arc-derived material has been accreted to the inner-trench wall and that the amount of oceanic crust incorporated into the subduction wedge was relatively minor. Directly west of the inner-trench wall, metamorphic rocks, marble and limestone identical to those dredged, crop out in a “blueschist belt” in northern Hispaniola. Behind the trench, to the south, low-grade metavolcanics in Puerto Rico appear to be continuous across Mona Passage to central Hispaniola. This belt parallels the higher-grade complex within the trench, and as such constitutes a paired metamorphic belt that extends for more than 500 km along the northern Caribbean plate boundary. The development of this belt is related to the inception of the Puerto Rico Trench in the middle to late Cretaceous due to the southerly subduction of the Atlantic plate beneath the Caribbean plate. Eastward movement of the Caribbean plate relative to the North American plate in the late Cretaceous and early Tertiary initiated transcurrent and tensional faulting which enhanced the uplift of the metamorphosed subduction wedge. Local tectonic readjustments which began in late Eocene time created the grabenlike trench observed at present and caused at least 3500 m of subsidence along the innertrench wall north of Puerto Rico, since Miocene time.

Magmatic Evolution of Semisopochnoi Island, Alaska: Trace-Element and Isotopic Constraints

Semisopochnoi has erupted a tholeiitic suite of moderately evolved lavas (

Oxygen isotopic evidence for meteoric water interaction with the Captains Bay pluton, Aleutian Islands

SiO_{2} = 50-65%; Zr = 64-250 ppm; La = 10-60x chondrites

The Occurrence and Origin of Andesites and Dacites From the Southern Juan de Fuca Ridge

), whose bulk-rock chemistry and phenocryst mineralogy are generally compatible with an important role for fractional crystallization in controlling the overall evolution of the suite. Trace-element data, however, are not consistent with crystal fractionation operating alone. Instead, a variety of plots involving incompatible elements (or ratios among them) demonstrate that simple mixing between basaltic and silicic components can reproduce the essential aspects of the trace-element variations and must have been operative as well. We are unable to distinguish between a silicic end-member represented by residual dacitic liquid like one of the analyzed lavas (therefore implying magma mixing) and one consisting of pre-existing silicic crustal rocks (therefore implying assimilation). Variations in Sr and Nd isotopic compositions are very limited: