Hervé Bellon

Cenozoic K-rich adakitic volcanic rocks in the Hohxil area, northern Tibet : Lower-crustal melting in an intracontinental setting

It is generally accepted that the Cenozoic potassic volcanic rocks of northern Tibet were derived from a lithospheric mantle source. Here we report new chronological, geochemical, and isotopic data for the Miocene (ca. 18-15 Ma) K-rich adakitic volcanic rocks from the Hohxil area of the Songpan-Ganzi block in northern Tibet. We contend that these rocks were generated by partial melting of the mafic lower crust, in an intracontinental setting unrelated to subduction of oceanic crust. The Hohxil rocks exhibit high Sr/Y and La/Yb ratios, high Sr and La contents, but low Yb and Y concentrations, similar to adakites formed by slab melting associated with subduction. However, their relatively low e N d values (-2.09 to -4.58); high 8 7 Sr/ 8 6 Sr (0.7072-0.7075), Th/U, Th/Ba, and Rb/Ba ratios; and distinctive potassium enrichments (K 2 O > Na 2 O) are very different from the composition of typical adakites. In addition, those K-rich adakitic rocks with the highest SiO 2 contents (>61 wt%) exhibit the lowest 8 7 Sr/ 8 6 Sr ratios and highest e N d values and are the oldest Cenozoic volcanic rocks exposed in the Songpan-Ganzi block, suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma. Taking into account the composition of lower-crustal mafic xenoliths in Tibet, as well as the tectonic and geophysical evidence, we conclude that the Hohxil adakitic magmas were produced by partial melting of amphibole-bearing eclogites with a K-rich mafic bulk composition, in the lower part (≥∼55 km) of the thickened northern Tibetan crust. Partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward-subducted continental lithosphere.

The geochemistry of young volcanism throughout western Panama and southeastern Costa Rica: an overview

Oblique aseismic subduction below western Panama and southeastern Costa Rica has produced Recent arc-related volcanism. The aseismicity is probably related to the subduction of relatively hot oceanic lithosphere. The volcanism throughout this region over the past 2 Ma has been quite distinct, consisting of felsic magmas (andesites to rhyolites but mainly dacites) with geochemical signatures suggesting a metamorphosed basaltic source. It is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge. Rocks with slab-melt geochemistries and associated with young subducted crust have been termed adakites elsewhere. The young adakite melts are sometimes associated with a few rare young high-Nb basalts, but there is no obvious genetic link between them through differentiation. High-Nb basalts may also be derived from the partial melting of the subducted oceanic crust. High-Nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the Catalina Schist, California. Alternatively, the high-Nb basalts may be partial melts of phlogopite-rich mantle that has previously reacted with adakite magmas. Eruption of adakites and high-Nb basalts was preceded by a 2-3 Ma period of relative quiescence. Prior to this, there was a 7 Ma period of calc-alkaline volcanism typical of the present-day magmatism (associated with a distinct Benioff zone) found throughout the Central American arc. The abrupt transition in volcanism with time from an early calc-alkaline sequence to a later adakite-high-Nb basalt sequence may record a change in the tectonic setting of western Panama and southeastern Costa Rica over the past 12 Ma.

Trace element and SrNdPb isotopic constraints on a three-component model of Kamchatka Arc petrogenesis

The Kamchatka arc (Russia) is located in the northwestern Pacific Ocean and is divided into three segments by major sub-latitudinal fault zones (crustal discontinuities). The southern (SS) and central (CS) segments are associated with the subduction of old Pacific lithosphere, whereas the northern, inactive segment (NS) was formed during westward subduction of young (< 15 Ma) Komandorsky Basin oceanic crust. Further segmentation of the arc is outlined by the development of the Central Kamchatka Depression (CKD) intra-arc rift, which is oriented parallel to the arc and is splitting the CS into the active Eastern Volcanic Front (EVF) and the largely inactive, rear-arc Sredinny Range. The NS volcanics (15-5 Ma) include calc-alkaline lavas, shoshonites, adakites, and Nb-enriched arc basalts. Isotopically all magma types share high 143Nd/144Nd ratios of 0.512976-0.513173 coupled with variable 87Sr/86Sr (0.702610-0.70356). NS lavas plot within or slightly above the Pacific MORB field on the Pb isotopic diagrams. The EVF volcanoes have more radiogenic 143Nd/144Nd (0.51282-0.513139) and 208Pb/204Pb (38.011–38.1310) than the NS lavas. CKD lavas display MORB-like Nd isotope ratios at slightly elevated 87Sr/86Sr values accompanied by a slightly less radiogenic Pb composition. Kamchatka lavas are thought to be derived from a MORB-like depleted source modified by slab-derived siliceous melts (adakites) and fluids (NS), or fluids alone (CS and SS). The NS and EVF lavas may have been contaminated by small fractions of a sedimentary component that isotopically resembles North Pacific sediment. Petrogenesis in the Kamchatka arc is best explained by a three-component model with depleted mantle wedge component modified by two slab components. Slab-derived hydrous melts produced incompatible element characteristics associated with northern segment lavas, while hydrous slab fluids caused melting in the depleted mantle below the southern and central segments of the Kamchatka arc. Trace element characteristics of Kamchatka lavas appear to be controlled by slab fluids or melts, while radiogenic isotope ratios which are uniform throughout the arc reflect depleted composition of sub-arc mantle wedge.

Initiation of subduction and the generation of slab melts in western and eastern Mindanao, Philippines

Adakite, found in both the eastern and western parts of Mindanao Island, Philippines, is a rare rock type, characterized by low heavy rare earth elements and Y contents together with high Sr/Y ratios, and is considered to be the result of the melting of young subducted oceanic crust, which leaves an eclogite residue. Pliocene-Quaternary adakites from western Mindanao (Zamboanga Peninsula) are probably derived from the melting of the young Miocene Sulu Sea crust, which is currently subducting beneath Zamboanga. Associated Nb-enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts. In eastern Mindanao, Pliocene-Quaternary cones and plugs of typical adakitic composition mark the trace of the Philippine fault in Surigao and north Davao. The underlying Philippine Sea crust is of Eocene age and therefore cannot melt under normal subduction thermal conditions. Thermal models indicate that melting at the start of subduction can occur. Subduction of the Philippine Sea plate began 3 to 4 Ma beneath eastern Mindanao and probably accounts for the presence of adakites along the Philippine fault.

Late Miocene adakites and Nb-enriched basalts from Vizcaino Peninsula, Mexico: Indicators of East Pacific Rise subduction below southern Baja California?

A typical slab melt association was emplaced from 11 to 8 Ma in the Santa Clara volcanic field, Vizcaino Peninsula, Baja California Sur. It includes adakitic domes and associated pyroclastic flow deposits, together with lava flows of niobium-enriched basalts. The trace element and isotopic (Sr-Nd-Pb) signatures of adakites are consistent with melting of altered mid-ocean ridge basalts, and the sources of the Nb-enriched basalts contain an enriched mantle wedge component. Such associations commonly form at depths of 70–80 km during low-dip subduction of very young oceanic crust. However, the Santa Clara field is relatively close (100 km) to the paleotrench, which suggests that the genesis of its adakites and Nb- enriched basalts occurred in a very high thermal regime linked to the subduction of the then-active Guadalupe spreading center of the East Pacific Rise. Our data suggest that the asthenospheric window documented below northern Baja California also developed beneath the south of the peninsula during the Neogene. This hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region.

Post-collisional transition from calc-alkaline to alkaline volcanism during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff

Abstract During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle.

Spatial and temporal evolution of basalts and magnesian andesites (“bajaites”) from Baja California, Mexico: the role of slab melts

Late Miocene to Quaternary basalts and associated magnesian basaltic andesites and andesites, locally referred to as ‘‘bajaites’’, occur in the central part of the Baja California (BC) Peninsula. They form five volcanic fields (Jaraguay, San Borja, San Ignacio, Santa Rosalia, La Purisima) delineating a 600-km-long array parallel to the Gulf of California. They range in age from Late Miocene to Pleistocene, and display very specific geochemical characteristics: SiO2=50% to 58%, high MgO contents, very low FeO*/MgO ratios usually less than 1.5, highly fractionated rare earth element patterns with low Yand heavy rare earth element, very high Sr (commonly between 2000 and 3000 ppm) and Ba (up to 2300 ppm) contents. The geochemical study and K–Ar dating of ca. 50 samples of these rocks allow us to show that most of their incompatible element ratios, which vary significantly in space and time, reflect source heterogeneities rather than partial melting, fractional crystallisation or crustal contamination effects. Their slab melt imprint increases from northwest to southeast and with time. It is best expressed in the geochemical signatures of Quaternary lavas from La Purisima volcanic field. These features reflect the origin of the ‘‘bajaites’’ by melting of mantle peridotites previously metasomatised by slab melts, in connection with the opening of an asthenospheric window below the Baja California Peninsula during Early and Middle Miocene in northern Baja California, and during Late Miocene in southern Baja California. Melting was initiated by the high thermal regime accompanying ridge subduction or slab tearing/breakoff, and later by Plio-Pleistocene thermal pulses linked to the opening of the Gulf of California. We show that the incongruent melting of metasomatic pargasitic amphibole, leaving a garnet-rich residue, accounts for most of the specific geochemical features of the magnesian andesite suite. This breakdown started at ca. 1000 jC at depths of 70–110 km, and amphibole was probably not entirely consumed during the melting process. D 2002 Elsevier Science B.V. All rights reserved.

Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano, Panama

The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.

Geochemical Diversity of Late Miocene Volcanism in Southern Baja California, Mexico: Implication of Mantle and Crustal Sources during the Opening of an Asthenospheric Window

Five main petrologic and geochemical groups can be identified among the Middle to Late Miocene lavas from the western part of southern Baja California: (1) calc‐alkaline and K‐rich andesites emplaced between 15.5 and 11.7 Ma; (2) adakites and (3) associated niobium‐rich basalts erupted between 11.7 and 8.5 Ma in the Santa Clara volcanic field, Vizcaino Peninsula; (4) 10.6–9.2 Ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near San Ignacio; and (5) magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11.7 Ma south of San Ignacio and between 9.7 and 8.8 Ma near La Purisima. These lavas, although spatially and temporally related, display very different geochemical signatures. Their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis. Partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface. These magmas were eventually trapped in the mantle wedge where they reacted with ultramafic lithologies. Such slab‐melt‐metasomatized mantle could then melt to produce niobium‐rich basalts or magnesian andesites, depending on the pressure that controlled the stability of garnet into the mantle wedge. The melting of fluid‐metasomatized mantle wedge led to the emplacement of andesites. In southern Baja California, the opening of a slab window following active ridge subduction resulted in the additional contribution of partial melts from the suboceanic mantle uprising through the tear in the slab. This process might be responsible for the occurrence of tholeiitic basalts and basaltic andesites near San Ignacio. The studied association can be considered as a modern analog of high‐thermal‐regime Archean subductions.

Magmatic response to abrupt changes in geodynamic settings: Pliocene—Quaternary calc-alkaline and Nb-enriched lavas from Mindanao (Philippines)

Abstract Mindanao, the largest island in the southern Philippine archipelago, is a composite of at least two terranes; one with Eurasian affinity (western Mindanao) and the other belonging to the Philippine Mobile Belt (eastern Mindanao), of Philippine Sea plate affinity. The island is surrounded by three subduction zones that have been installed only in the past 4 m.y. Prior to this, the two terranes were separated by an ocean that disappeared continuously by subduction of its two edges beneath western and eastern Mindanao, where mostly typical arc magmatic rocks, dated at 30 Ma, 19−15 Ma, 12−11 Ma and 7−4 Ma were emplaced. The suturing of the two terranes occurred at ca. 5 Ma. Following this major structural reorganization, abrupt changes are recorded in the The geochemical diversity of magmatic types in Mindanao is attributed to: 1. (1) the highly heterogenous character of their mantle source, which contains variable amounts of metasomatic pargasite and phlogopite, and, possibly, an additional OIB component that could contribute to Nb enrichment of NEB; 2. (2) the contribution of melts from the ubducted oceanic crust; these melts are either emplaced directly on the surface (adakites) or act as metasomatic agents leading to a Nb-enriched mantle, a probable source of NEB. Garnet and amphibole fractionation could also account for additional variations in the MREE and the HREE.