The role of water in the petrogenesis of Mariana trough magmas

The generation of large deposits of Au and Cu in suprasubduction-zone settings depends upon a combination of factors, including availability of the chalcophile elements to arc magmas in their mantle source regions, and the operation of suitable hydrothermal systems in the upper crust where the deposits eventually form. The removal of chalcophile elements from the mantle wedge into arc magmas can only occur if sulfide is absent from the melted source rock, requiring oxidation of the mantle wedge to values of log f O 2 > FMQ + 2, where f O 2 is oxygen fugacity and FMQ is the fayalite-magnetite-quartz oxygen buffer. The only agent capable of effecting this change is ferric iron, carried in solution by slab-derived partial melts or supercritical fluids. Arc magmas with high potential to generate Au and Cu deposits will have certain geochemical characteristics; they will have f O 2 more than two log units above FMQ and they will have either adakitic, sodic-alkaline, or potassic-ultrapotassic affinities. Favorable tectonic settings include subduction of very young lithosphere or very slow or oblique convergence, flat subduction, and the cessation of subduction.

Roasting the mantle: Slab melting and the genesis of major Au and Au-rich Cu deposits

Osmium isotopic compositions of mantle xenoliths: A global perspective

https://www.utdallas.edu/~rjstern/pdfs/crustalevolution2002.pdf

Crustal evolution in the East African Orogen: a neodymium isotopic perspective

material in or, most probably, below the asthenospheric mantle, Petrological and geochemical data are presented for a suite of brought down in a subducting slab at a former convergent plate ultramafic xenoliths (lherzolites and wehrlites) from La Grille margin. volcano, Grande Comore island. These xenoliths have peculiar textural features characterized by reaction rims or glassy patches, where clinopyroxene grows at the expense of orthopyroxene. The glasses vary from very strongly silica-oversaturated to strongly undersaturated varieties. They are generally homogeneous within a single xenolith, are extremely alkali rich, with Na2O contents (up

Carbonatite Metasomatism of the Oceanic Upper Mantle: Evidence from Clinopyroxenes and Glasses in Ultramafic Xenoliths of Grande Comore, Indian Ocean

Much of northern and central Mexico underwent east-northeast extension in the mid- to late Cenozoic; this area constitutes a major but little-recognized part of the Basin and Range province. The extended region is bounded on the east by the Laramide thrust front of the Sierra Madre Oriental. On the west, the relatively unfaulted Sierra Madre Occidental separates the extended area in central Mexico from that around the Gulf of California. Extension occurred as far south as what is now the Trans-Mexican volcanic belt, and in Oaxaca, south of the belt. The Basin and Range province in Mexico constitutes approximately half of the 19 x 10 5 km 2 of western North America that underwent mid- to late Cenozoic extension. North-northwest orientations of numerous epithermal vein systems indicate that east-northeast extension began as early as 30 Ma in areas north of the volcanic belt. Major episodes of faulting began at about 23 to 24 Ma and 12 to 13 Ma, both in Mexico and in the southwestern United States. Faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting. Widespread Quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the Trans-Mexican volcanic belt. In contrast, the tectonics in and south of the belt are now probably related to subduction of the Rivera plate. Contemporaneity of the 12 Ma episode with early extension around the Gulf of California attributed to Pacific-North American plate boundary reorganization suggests that extension is related predominantly to plate boundary effects. The beginning of extension at ∼30 Ma may be related to initial encroachment of the East Pacific Rise upon the trench that lay off western North America.

The real southern Basin and Range: Mid- to late Cenozoic extension in Mexico

Plate interactions control middle–late Miocene, proto-Gulf and Basin and Range extension in the southern Basin and Range

Twenty-six spinel peridotite xenoliths were studied from five QuaINTRODUCTION ternary volcanic fields that define a transect extending for ~1700 km The petrological and geochemical evolution of North from southeast to northwest across the southern Basin and Range America’s lithospheric mantle is pieced together from Province in Mexico. Calculated equilibration temperatures and studies of mafic volcanic rocks and their mafic–ultramafic pressures are highest for xenoliths from Ventura–Espiritu Santo xenoliths. During Cenozoic time, mantle-xenolith-bear(1073±41°C, 19·6±3·0 kbar; n=9) and Durango ing magmas have erupted throughout the western part (1082±21°C, 17·0±1·7 kbar; n=7). These fields lie near of the continent, from Alaska to Mexico. Comprehensive the center of the transect in the interior of the Tepehuano tectonic petrological and geochemical studies have been conterrane and have xenoliths dominated by coarse-equant and coarseducted on mantle xenoliths from numerous localities in tabular textures. Spinel–pyroxene clusters, interpreted as dethe western contiguous USA (Wilshire & Shervais, 1975; compression reaction products of garnet+olivine, are only found at Frey & Prinz, 1978; Reid & Woods, 1978; Basaltic these fields, only in xenoliths with calculated pressures >15 kbar. Volcanism Study Project, 1981; Menzies et al., 1987; In contrast, calculated temperatures and pressures are lowest for the Wilshire et al., 1988; Galer & O’Nions, 1989) and Canada Santo Domingo (1026±19°C, 14·4±2·0 kbar; n=4) and (Littlejohn & Greenwood, 1974; Sinclair et al., 1978; Fujii San Quintin (1034 ± 66°C, 10·7±2·5 kbar; n=5) fields, & Scarfe, 1982; Francis, 1987; Canil & Scarfe, 1989; where xenoliths display porphyroclastic or granuloblastic textures. Canil et al., 1990). In comparison, studies of xenoliths from These two fields mark the ends of our transect and lie near tectonic Mexico have been relatively few. Important contrasts are terrane boundaries. Calculated oxygen fugacities, expressed relative likely between the Mexican lithospheric mantle and that to the fayalite–magnetite–quartz buffer (DFMQ), increase from farther north, because no Archean rocks are known in southeast to northwest along the transect: Santo Domingo, Mexico and it appears to have had a distinct tectonic –1·3±0·7 (n=4); Ventura–Espiritu Santo, –0·6±0·4 (n= history from the rest of North America over much of 9); Durango, –0·2±0·3 (n=7); Mesa Cacaxta, 0·3 (n=1); geological time (Sedlock et al., 1993). San Quintin, 0·0±0·2 (n=5). The lithosphere closest to the Aranda-Gomez & Ortega-Gutierrez (1987) sumMesozoic–Cenozoic paleotrench appears to have been most strongly marized the known mantle xenolith localities of Mexico affected by slab-derived, oxidized fluids or melts. (Fig. 1). Like the mantle xenolith suites described from

Mexican Peridotite Xenoliths and Tectonic Terranes: Correlations among Vent Location, Texture, Temperature, Pressure, and Oxygen Fugacity

The San Quintin Volcanic Field (SQVF) is unique for the Baja California peninsula as the only known location of intraplate-type mafic alkalic volcanism and the only known source of peridotitic and granulitic xenoliths. It consists of 10 distinct Quaternary volcanic complexes. The oldest cones mainly erupted primitive magmas (Mg # > 64)(Mg # = 100 × Mg/(Mg + (0.85 × FeTotal))), which carried occasional small xenoliths. As the SQVF evolved with time, differentiated magmas (Mg # < 64) became increasingly common, but primitive magmas, virtually devoid of xenoliths and unusually rich in olivine phenocrysts, dominanted at the youngest cones. Abundances of incompatible elements declined during evolution of the SQVF, implying a temporal increase in the extent of partial melting in the mantle, or progressive exhaustion of these elements in the source. Samples from two cones, Mazo and Ceniza, show relatively low Ce/Pb, eNd, and 206Pb/204Pb and high 87Sr/86Sr, which we interpret as evidence for crustal contamination of these magmas. Small isotopic variations for the other cones are collectively interpreted to reflect involvement of at least three mantle components beneath the SQVF. Ranges in isotopic composition overlap for primitive and differentiated rocks, supporting fractional crystallization as the mechanism for deriving the latter from the former. Most differentiated rocks can be successfully modeled by fractional crystallization of olivine, plagioclase, clinopyroxene, and spinel from primitive parents. The largest and most abundant xenoliths were carried by differentiated magmas, indicating that fractional crystallization took place within the mantle, below the level of peridotite entrainment, and reflecting the importance of fractionation-elevated volatile contents for driving these differentiated magmas rapidly to the surface. Primitive rocks of the SQVF are unusual compared to other reported intraplate-type mafic alkalic suites from around the world in having relatively high Al2O3 and Yb, as well as low La/Yb and CaO/Al2O3. These characteristics and trends of rising Al2O3 and falling CaO with decreasing incompatible element abundances are all consistent with origins for the SQVF primitive magmas by progressive partial melting of spinel lherzolite at unusually shallow levels in the mantle.

San Quintín Volcanic Field, Baja California Norte, México: Geology, petrology, and geochemistry

The Guanajuato region, located in the southern part of the Basin and Range tectonic province, is characterized by a rhombohedral fault pattern. Extensional faulting-as well as related uplift, erosion, and deposition of continental clastic deposits-was in progress at 49 Ma, as indicated by K-Ar dating of intercalated mafic volcanic lavas. Earliest extension is registered in the lower member of the Guanajuato conglomerate and produced NW-trend-ing, down to the southwest, normal faults and northeast tilting of the hanging-wall clastic sequence. Beds in the upper member of the Guanajuato conglomerate display lateral thickness variations, changes in size and composition of the clasts, and ENE and SE inclination, all of which suggest activity along a second set of NE-trending younger faults. Extension continued also after emplacement of the overlying Bufa ignimbrite (∼37 Ma, K-Ar). Dip differences between the conglomerate and the Calderones Formation ( 30 Ma), which is part of the volcanic sequence that...

Paleogene Extension in the Southern Basin and Range Province of Mexico: Syndepositional Tilting of Eocene Red Beds and Oligocene Volcanic Rocks in the Guanajuato Mining District

José Jorge Aranda-Gómez

Papers

The real southern Basin and Range: Mid- to late Cenozoic extension in Mexico

Plate interactions control middle–late Miocene, proto-Gulf and Basin and Range extension in the southern Basin and Range

Mexican Peridotite Xenoliths and Tectonic Terranes: Correlations among Vent Location, Texture, Temperature, Pressure, and Oxygen Fugacity

San Quintín Volcanic Field, Baja California Norte, México: Geology, petrology, and geochemistry

Paleogene Extension in the Southern Basin and Range Province of Mexico: Syndepositional Tilting of Eocene Red Beds and Oligocene Volcanic Rocks in the Guanajuato Mining District