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Whether the downgoing slab in modern subduction zones is able to melt is an important unresolved aspect of plate tectonics. Rare arc lavas with high Sr/Y ratios and low Y and heavy rare earth element concentrations, called adakites, are often attributed to melting of young subducted crust in subduction zones. Osmium isotopic data obtained for 13 adakite samples from Mindanao, Philippines, conflict with the slab-melting model. Ten of the samples have unradiogenic [sup.187Os]/[sup.188Os] ratios inconsistent with partial melting of the ca. 50 Ma Philippine Sea lithosphere subducting beneath Mindanao. The Os isotope signatures are similar to those of young mid-oceanic-ridge basalts and normal arc rocks, consistent with a mantle source. In addition to implications for mass transfer in subduction zones, the apparent association of adakites with Cu-Au deposits makes understanding their origin, and that of the associated metals, economically interesting. This study suggests that a reevaluation is required of the slab-melting hypothesis in general, and that the metals associated with the Mindanao adakites come from the mantle. Keywords: adakites, osmium, Philippines, subduction.

It was first suggested by R.W. Kay that adakites may represent melts of subducted slab: since then, the term adakite has become synonymous with slab melts based on their unusual geochemical signature. This contribution, using the Northern volcanic zone of the Andes as an example, aims (1) to expose the weakness in simply associating a geochemical signature with a genetic mechanism and (2) to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting. We conclude that although slab melting probably occurs in some arcs, regional geochemical trends in the Northern volcanic zone and their relationship to the subduction-zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge-derived basaltic magmas. Keywords: Ecuador, subduction, adakite, Northern volcanic zone, slab melting.

The assumption that the isotopic ratios of anatectic melts generated in response to crust-magma interaction are equivalent to the bulk source rock from which they are derived was tested experimentally via dehydration melting of a Proterozoic biotite granite. Heating of the coarse-grained granite at 1250 [degrees] C for durations of 1 to 48 h produced extreme disequilibrium among simultaneously generated mafic and felsic melts (quenched to glass) and whole-rock 87Sr/86Sr ratios. In-situ microdrill Sr isotopic analyses across gradational contacts between mafic (high-87Sr/86Sr) and felsic (low-87Sr/86Sr) glass register significant isotopic and chemical heterogeneity on the submillimetre scale. Our experiments suggest that during the early stages of wall-rock melting, when the rate of heating may exceed melting rates, isotopic equilibrium may not be maintained, thus producing contaminant liquids having isotopic compositions different from the bulk source rock. If isotopic equilibrium is not attained during partial melting in the crust, current geochemical models of open-system magmatic processes may require modification.

Neogene through Pleistocene lava flows of the Nevados de Payachata region (lat 18 degrees S) on the Altiplano of northern Chile fall into two discrete chemical groups defined by age and incompatible element concentrations. The Neogene suite (10.5-6.6 Ma) has trace element concentrations comparable to arc magmas erupted on thin crust in central Chile. Pleistocene lava flows (0.29-0 Ma) are enriched in incompatible elements relative to Neogene samples but have similar Sr, Nd, and Pb isotopic ratios. Incompatible element enrichment in mafic rocks reflects deep-crustal or subcrustal processes. Neogene volcanism in northern Chile immediately followed a period of intense crustal thickening. Uplift rates accelerated at approximately 15 Ma, indicating that the 'thin-crust'-type Neogene magmas actually traversed a thickened crust. The isotopic and trace element relations are the result of contamination of mantle-derived basalts in an upwardly growing lower-crustal interaction zone, defined to be the part of the lower crust at or above its solidus. Because the lower crust reaches postorogenic thermal equilibrium slowly, the lower-crustal interaction zone during Neogene magmatism had essentially the same thickness as it did prior to deformation. Thermal relaxation of the lower crust after thickening produced an upwardly growing column of crust near its solidus. Only 15 m.y. after crustal thickening did ascending, mantle-derived mafic magmas encounter previously unmelted, fertile crust near its solidus in the upper fringes of the interaction zone. Here the magmas were contaminated with incompatible element-enriched crustal melts, forming the parental magmas for the Pleistocene suite.

Lavas from Quaternary minor volcanic centers behind the Central Andean volcanic arc in Bolivia form a compositional continuum with rocks from the arc front, extending to more primitive compositions. The lavas show petrographic and geochemical evidence for deep crustal contamination during differentiation. Sr, Nd, and Pb isotopic data from centers within 30 km of each other encompass the entire range found in the Central Volcanic zone of the Andes. The most mafic rocks are largely typical of arc magmas elsewhere, and may represent parental magmas to the porphyritic andesites and dacites erupted from stratovolcanoes along the arc to the west. Andean arc magmas therefore undergo considerable contamination during ascent through the thick crust. The only basalts lacking petrographic evidence for contamination are picrobasalts from Chiar Kkollu that form the most primitive end member of the compositional spectrum. These rocks resemble ocean-island basalts in some respects, indicating that variations in crustal contamination across the strike of the arc may be superimposed on variations in the mantle source composition.

Published K-Ar ages of laccoliths of the Henry and La Sal Mountains range from 23 to 56 Ma, with a bimodal distribution in the Eocene and late Oligocene epochs. Eocene ages require a period of magmatism during the Laramide Orogeny, a time of little or no igneous activity in the western United States at these latitudes. On the basis of 40Ar/39Ar and fission-track ages, we conclude that intrusive activity occurred during middle to late Oligocene and early Miocene time (31-20 Ma) in the Henry, La Sal, and Abajo Mountains. These results imply a genetic link to magmatism in the Reno-Marysvale and San Juan volcanic fields, which lie to the west and east of the Colorado Plateau, respectively. We interpret the small volume of the Iaccoliths, relative to the surrounding volcanic fields, to reflect differences in crustal composition and tectonic history of the plateau and adjacent areas. Eocene K-Ar ages result from excess 40Ar or contamination by xenocyrsts. Excess argon is manifested by trapped argon with 40Ar/36Ar ratios greater than atmosphere (295.5), resulting in total-gas or apparent K-Ar ages that are too old. Laser-fusion experiments and microprobe data show that some samples with Eocene or older total-gas ages and uncorrelated inverse correlation plots are affected by radiogenic argon residing in incompletely outgassed hornblende xenocrysts. All such samples contain middle- to late-Oligocene magmatic hornblende crystals as revealed by laser-fusion experiments. Xenocrysts may be derived from amphibolite xenoliths common in these rocks. Mixing calculations indicate that only 1%-4% contamination may be required to increase K-Ar ages by 10-15 Ma for xenocrysts outgassed as much as 75%. The 40Ar/39Ar method is capable of identifying small degrees of magmatic contamination by older K-bearing phases.

The locality of the Chiar Kkollu was found to be of early Miocene age based on evidences provided by radioactive dating, field observations and paleomagnetic proofs. The discovery, which was the result of the study made on the locality's crust, crustal contamination and the magma genesis of the volcanic rocks, also provided evidences to support the tectonic evolution of the Bolivian Andes. Further studies on the chemistry and isotopic composition of Chiar Kkollu need to be undertaken to determine the locality's central volcanic zone processes.

The Tatara-San Pedro volcanic complex, a Quaternary to Holocene composite volcano in central Chile, is underlain by late Miocene (6 Ma) plutons and Tertiary metavolcanic rocks. The plutons were intruded into greenschist facies metavolcanic rocks at a depth of about 4-5 km. Together, these rocks reveal important information about the development of continental arcs. In some cases, apparent eruptive ages of metavolcanic rocks are younger than ages of intruding plutons, indicating that there was (1) pervasive reheating of metavolcanic rocks or (2) that metavolcanic rocks were locally derived and intruded by their own magma chambers (plutons). We have estimated uplift or denudation rates of nearly 1 mm/yr since late Miocene time. Normal faulting currently dominates the brittle behavior of the upper crust; therefore, uplift is probably related to magmatic addition to the crust rather than to compression and shortening. Although plutonic and metavolcanic rocks display expected arc geochemical affinities, the isotopic similarity of metavolcanic and plutonic rocks to Quaternary volcanic rocks is strong; this makes it difficult to use subvolcanic rocks as upper crustal contaminants to evaluate the igneous evolution of the Quaternary deposits. Most plutonic, metavolcanic, and Quaternary volcanic rocks exhibit the isotopic characteristics of juvenile crust. Some samples, however, exhibit sparse but unmistakable evidence (87Sr/S6Sr and 40Ar/39Ar isotopic data) for the presence of older, probably Precambrian through Triassic radiogenic crust beneath the volcano. Crustal structure in this arc can be described by a three-layer model in which a central section of Precambrian through Triassic rock is underplated by juvenile material, is intruded by juvenile material, or has juvenile material transported through it to be emplaced at or near the surface. Continued erosion of juvenile material at the surface may be balanced to some degree by continued volcanism and epizonal plutonism.