[1] A new ICP-MS database for glasses from the Mariana Trough, together with published and new ICP-MS data from the Mariana arc, provides the basis for geochemical mapping of the Mariana arc-basin system. The geochemical maps presented here are based on the graphic representation of spatial variations in geochemical proxies for the principal mantle and subduction components. The focus is on three elements with high and similar partition coefficients but different behavior in subduction systems, namely, Ba, Th, and Nb. Two elements with different partition coefficients, Ta and Yb, are used as normalizing factors. Ratio maps (Ta/Yb, Nb/Ta, Th/Ta, Ba/Ta, Ba/Th) provide the simplest petrogenetic insights, subduction zone addition maps based on deviations from a MORB array provide more quantitative insights, and component maps represent an attempt to isolate the different subduction components. The maps shown here indicate the presence of a variably depleted asthenosphere and three added components: a Nb-Th-Ba component, a Th-Ba deep-subduction component, and a Ba-only shallow-subduction component. The asthenosphere entering the system is enriched relative to N-MORB and appears to be focused at three sites within the Mariana Trough. The Nb-Th-Ba component is present mainly in the north of the arc (the Northern Seamount province and northern Central Island Province), the northern edge of the Mariana Trough, and two locations within the Southern Seamount Province. It has a distinctively high Nb/ Ta ratio and a moderate enrichment in Th and Ba relative to Nb. Its composition and distribution indicate that it may not be part of the present subduction system but instead originates in mantle lithosphere previously enriched above the subduction zone by addition of small-degree, subduction-modified mantle melts. The Th-Ba component is present throughout the arc and, in minor amounts, in parts of the back-arc basin. The Ba-only component is mainly present in the central part of the arc and at the edges of the backarc basin. Overall, the geochemical maps provide a new perspective on the geochemical processes that accompany the evolution of an arc basin system from prerifting lithospheric enrichment, through arc-rifting to arc volcanism and back-arc spreading.
The southern Mariana Arc-Trench system is rapidly deforming, resulting in unusual interactions between arc and back-arc basin (BAB) magmatic systems. New geochemical data for volcanoes in this region are presented and explored. Tracey Seamount, an extinct submarine volcano about 30 km northwest of Guam, is the southernmost stratovolcano of the Mariana Arc. Tracey is built about 125 km above the subducted slab and has erupted a bimodal suite of typical arc mafic and felsic lavas as recently as 0.527 Ϯ 0.023 Ma ( 40 Ar/ 39 Ar age). An unusual cluster of small basaltic volcanoes, informally termed the Alphabet Seamount Volcanic Province (ASVP), is found about where the next arc volcano to the southwest of Tracey Seamount should have grown. Samples from six of these volcanoes were studied here. At least two ASVP volcanoes were recently active, as shown by hydrothermal activity. The lack of magmatic focusing to build a single stratovolcano where the ASVP is situated reflects strong extension in the BAB. Construction of northern ASVP volcanoes is controlled by east-west extension accompanying opening of the Mariana Trough. In contrast, southern ASVP volcanoes are affected by north-south extension due to rapid rollback of a narrow slab of Pacific seafloor that is subducted along the east-west trending Challenger Deep segment of the Mariana Trench to the south. ASVP lava compositions are distinct from Tracey Seamount and other Mariana Arc lavas, instead showing affinities with Mariana Trough BAB basalt (BABBs): they are mafic, tholeiitic, low-K 2 O and LREE-depleted, with low 87 Sr/ 86 Sr, but show a subtle gradient from somewhat more arc-like lavas closer to the trench to BABBs farther west. The unusual tectonic setting of ASVP provides a unique perspective on how different arc magma batches reflect melting of mantle with strong compositional gradients which are mixed together beneath long-lived arc volcanoes but here rise to form scattered small ASVP volcanoes.
Major and trace element analyses are presented for 110 samples from the DSDP Leg 60 basement cores drilled along a transect across the Mariana Trough, arc, fore-arc, and Trench at about 18°N. The igneous rocks forming breccias at Site 453 in the west Mariana Trough include plutonic cumulates and basalts with calc-alkaline affinities. Basalts recovered from Sites 454 and 456 in the Mariana Trough include types with compositions similar to normal MORB and types with calc-alkaline affinities within a single hole. At Site 454 the basalts show a complete compositional transition between normal MORB and calc-alkaline basalts. These basalts may be the result of mixing of the two magma types in small sub-crustal magma reservoirs or assimilation of calc-alkaline, arc-derived vitric tuffs by normal MORB magmas during eruption or intrusion.A basaltic andesite clast in the breccia recovered from Site 457 on the active Mariana arc and samples dredged from a seamount in the Mariana arc are calc-alkaline and similar in composition to the basalts recovered from the Mariana Trough and West Mariana Ridge. Primitive island arc tholeiites were recovered from all four sites (Sites 458-461) drilled on the fore-arc and arc-side wall of the trench. These basalts form a coherent compositional group distinct from the Mariana arc, West Mariana arc, and Mariana Trough calc-alkaline lavas, indicating temporal (and perhaps spatial?) chemical variations in the arc magmas erupted along the transect.Much of the 209 meters of basement cored at Site 458 consists of endiopside-and bronzite-bearing, Mg-rich andesites with compositions related to boninites. These andesites have the very low Ti, Zr, Ti/Zr, P, and rare-earthelement contents characteristic of boninites, although they are slightly light-rare-earth-depleted and have lower MgO, Cr, Ni, and higher CaO and AI2O3 contents than those reported for typical boninites. The large variations in chemistry observed in the lavas recovered from this transect suggest that diverse mantle source compositions and complex petrogenetic process are involved in forming crustal rocks at this intra-oceanic active plate margin.
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