Mark S. Drummond

Petrogenesis of slab-derived trondhjemite–tonalite–dacite/adakite magmas

The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1–32 kbar) and temperatures (700–1150°C) have shown that melt compositions are primarily trondhjemite–tonalite–dacite (TTD). High-Al (> 15% Al 2 O 3 at the 70% SiO 2 level) TTD melts are produced by high-pressure (≥ 5 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxene ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young ( −1 ) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge. The minimum P–T conditions for slab melting are interpreted to be 22–26 kbar (75–85 km depth) and 750–800°C. This P–T regime is framed by the hornblende dehydration, 10°C/km, and wet basalt melting curves and coincides with numerous potential slab dehydration reactions, such as tremolite, biotite + quartz, serpentine, talc, Mg-chloritoid, paragonite, clinohumite and talc + phengite. Involvement of overthickened (>50 km) lower continental crust either via direct partial melting or as a contaminant in typical mantle wedge-derived arc magmas has been presented as an alternative to slab melting. However, the intermediate to felsic volcanic and plutonic rocks that involve the lower crust are more highly potassic, enriched in large ion lithophile elements and elevated in Sr isotopic values relative to Cenozoic adakites. Slab-derived adakites, on the other hand, ascend into and react with the mantle wedge and become progressively enriched in MgO, Cr and Ni while retaining their slab melt geochemical signature. Our studies in northern Kamchatka, Russia provide an excellent case example for adakite-mantle interaction and a rare glimpse of trapped slab melt veinlets in Na-metasomatised mantle xenoliths.

Mount St. Helens: Potential example of the partial melting of the subducted lithosphere in a volcanic arc

Mount St. Helens, 50 km to the west of Mount Adams and the main Cascade volcanic chain, is only 80 km above the subducting oceanic lithosphere. The elevated temperatures off the subducting slab, because of the close proximity of the Juan de Fuca Ridge to the trench,may induce slab melting at a depth of ∼80 km. Dacites from Mount St. Helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure, i.e., relatively high AI (Al2O3 > 15% at 70% SiO2), low Y and Yb (because of garnet and amphibole stability in the source), low Sc, and high Sr and Eu. Trace element modeling of the partial melting of mid-oceanic ridge basalt (MORB) from the Juan de Fuca Ridge that yields a hornblende eclogite residue can reproduce the Mount St. Helens data (results off the model are quite distinct from data derived from the Mount Adams volcanic rocks). In contrast, Mount Adams is ∼135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate. The Cascade are has been active in its present locality, because of oblique subduction, for the past 7 m.y. The major volcanoes along the arc have existed for at least 500 ka, but Mount St. Helens has existed for <40 ka. We suggest that the subducting plate may have reached elevated temperatures, because of the approach of North America to the Juan de Fuca Ridge, at ∼40 ka, which initiated melting of the slab.

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.

Progressive enrichment of island arc mantle by melt-peridotite interaction inferred from Kamchatka xenoliths

Abstract The Pliocene (7 Ma) Nb-enriched arc basalts of the Valovayam Volcanic Field (VVF) in the northern segment of Kamchatka arc (Russia) host abundant xenoliths of spinel peridotites and pyroxenites. Textural and microstructural evidence for the high-temperature, multistage creep-related deformations in spinel peridotites supports a sub-arc mantle derivation. Pyroxenites show re-equilibrated mosaic textures, indicating recrystallization during cooling under the ambient thermal conditions. Three textural groups of clinopyroxenes exhibit progressive enrichment in Na, Al, Sr, La, and Ce accompanied by increase in Sr/Y, La/Yb, and Zr/Sm. Trace elements in various mineral phases and from felsic veins obtained through ion microprobe analysis suggest that the xenoliths have interacted with a siliceous (dacitic) melt completely unlike the host basalt. The suite of xenoliths grade from examples that display little evidence of metasomatic reaction to those containing an assemblage of minerals that have been reproduced experimentally from the reaction of a felsic melt with ultramafic rock, e.g., pargasitic amphibole, albite-rich plagioclase, Al-rich augite, and garnet. The dacitic veins within spinel lherzolite display a strong enrichment in Sr and depletion in Y and the heavy rare earth elements (e.g., Yb). The dacites are comparable to adakites (melts derived from subducted metabasalt), and not typical arc melts. We believe that these potential slab melts were introduced into the mantle beneath this portion of Kamchatka subsequent to partial melting of a relatively young (and hot) subducted crust. Island arc metasomatism by peridotite-slab melt interaction is an important mantle hybridization process responsible for arc-related alkaline magma generation from a veined sub-arc mantle.

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.

Insights into the volcanic arc mantle wedge from magnesian lavas from the Kamchatka arc

Active volcanism in the Kamchatka arc occurs where the Pacific Plate subducts beneath the Kamchatka peninsula south of its junction with the Aleutian arc. Most volcanism occurs within the Central Kamchatka Depression (CKD), a large graben oriented parallel to the trench, and along the Eastern Volcanic Front (EVF), located south and east of the CKD and closer to the trench. Differentiation trends range from calc-alkaline to tholeiitic. Fractionation of a mineral assemblage including olivine, clinopyroxene, and orthopyroxene, produces the tholeiitic trend, whereas separation of amphibole and magnetite, along with possible crustal assimilation, produces the calc-alkaline trend. A suite of near-primitive high-Mg basalts provides geochemical records of mantle sources and processes unobscured by differentiation. Rare earth element (REE) patterns range from slightly depleted ((Ce/Yb) n =0.8-1.5) to slightly enriched ((Ce/Yb) n =1.5-3.5). Rocks with the depleted REE patterns occur at the volcanic front in regions where a volcano or volcanic chain exists behind the volcanic front. Lavas with relatively enriched REE patterns occur behind the volcanic front and along portions of the volcanic front where behind-the-front volcanism is absent. Modeling of trace element abundances normalized to 10% MgO indicates that the rocks with the depleted REE patterns are derived from a more depleted source, inferred to represent refractory source material remaining after a previous generation of melt extraction within the arc. Mantle source material apparently convects into the mantle wedge from the rear, producing relatively enriched magmas when it melts for the first time. Relatively depleted magmas are produced if a second period of melting ensues as the mantle reaches the volcanic front.

On the Tectonic Significance of Arc Volcanism in Northern Kamchatka

The Vyvenka volcanic field records a period of Neogene, subduction-related volcanism in northern Kamchatka. Most models describing the tectonic evolution of the northwest Pacific do not account for this type of Neogene volcanism because the main locus of Pacific/Kula-North American convergence switched to the Aleutian Ridge during Eocene time. The Vyvenka volcanism, as well as oceanic spreading and crust formation within the Komandorsky Basin, demonstrate that this region remained tectonically and volcanically active in Neogene times. We report petrologic, geochemical, and K-Ar age data for the ~15 Ma Golovin and 6-8 Ma Valovayam volcanic rocks, two andésite suites within the Vyvenka volcanic field. The Golovin suite consists of medium- to high-K andesites with strong arc-like trace-element signatures, while the Valovayam suite consists of medium-K andesites with weaker arc-like trace-element signatures. The Valovayam andesites also contain some trace-element ratios indicative of melting of the subducted oceanic crust. These include high Sr/Y (30-50) and Zr/Sm greater than the chondritic value of 28. The Golovin andesites have overlapping Sr/Y (25-45) and lower Zr/Sm. The compositional differences between the Golovin and Valovayam andesites correlate with Neogene tectonic evolution of the Komandorsky region. In northern Kamchatka, subduction waned as spreading stopped in the Komandorsky Basin and newly generated oceanic crust entered the subduction zone. Thus, the trace-element signals of slab melts in the younger Valovayam rocks indicates melting of the young, hot Komandorsky Basin crust that entered the subduction zone and subsequent metasomatism of the mantle wedge. The weaker subduction signature of the Valovayam suite, which distinguishes it from the Golovin suite, records the decreasing vigor of subduction processes with time.

An example of trondhjemite petrogenesis: the Blakes Ferry pluton, Alabama, U.S.A.

Abstract The Blakes Ferry pluton is a calc-alkaline pluton that is exposed in southwestern Randolph County, Alabama, U.S.A. The petrogenesis of the Blakes Ferry pluton has been a controversy for almost 15 years. A petrogenetically coherent model is presented, however, as a result of new mineral analyses and the examination of previous analytical data. Crystal fractionation of plagioclase has been substantiated based on mass-balance calculations. Assimilation-crystal fractionation modelling and mixing calculations suggest that the biotite-muscovite-sphene-rich schlieren observed within the Blakes Ferry pluton are remnants of partially assimilated metasedimentary rock. Furthermore, the biotite chemistry of the Blakes Ferry pluton is quite similar to that of the biotites in the Wedowee schists which are the host rock to the Blakes Ferry intrusion. The above indicates the assimilation-mixing of the Wedowee metasediments occurred toward the end of plagioclase fractionation. The Blakes Ferry pluton has both l-and S-type characteristics interpreted as inherited from the source melting of an igneous rock with subsequent assimilation of the metasediments surrounding the pluton. The Elkahatchee Quartz Diorite, a large older tonalitic intrusive near the Blakes Ferry pluton, has been ruled out as a potential source rock contrary to previous interpretations. Crystal fractionation of a more mafic magma (e.g., gabbro) to generate the Blakes Ferry melt also does not seem plausible because the associated sequence of differentiated products are not observed in the region. Based on REE modelling and experimental work, the only plausible source is the partial melting of a MORB with a hornblende-quartz eclogite residual. The most likely source for the partial melting of a MORB is in a subduction related environment. Several researchers have proposed an arc setting for associated rocks within the same structural and metamorphic block that the Blakes Ferry poluton is situated. This suggests that the necessary setting for the partial melting of MORB was present in the region. The partial melting of approximately 28% MORB coupled with the generation of 72% hornblende-quartz eclogite residual in a subduction zone is envisioned. The density differences between the trondhjemitic melt and the surrounding mantle generated the ascent of the magma. Plagioclase fractionation via filter pressing probably occurred during ascent. The magma stalled in the Wedowee metasediments because the density contrasts no longer existed between melt and surrounding rocks. Assimilation and further fractional crystallization took place as the magma cooled to become the trondhjemitic schlieren-rich body observed in the field today.