Marc J. Defant

Origin of Mesozoic adakitic intrusive rocks in the Ningzhen area of east China: Partial melting of delaminated lower continental crust?

To the best of our knowledge, modern adakites have not been documented in a nonarc environment. We report geochemical and isotopic data for Early Cretaceous Anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction. The Anjishan adakitic intrusive rocks, which are exposed in the Ningzhen area of east China, have high Sr/Y and La/Yb ratios coupled with low Yb and Y as well as relatively high MgO contents and Mg numbers (Mg#; 0.4-0.6), similar to products from slab melting. However, low ∈ N d ( t ) values (-6.8 to-9.7) and high ( 8 7 Sr/ 8 6 Sr) i (0.7053-0.7066) are inconsistent with an origin by slab melting. The tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust. High Sr/Y and La/Yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting, implying that the crustal thickness exceeded 40 km in the Early Cretaceous. The present thickness of the crust in the Ningzhen area is only 30 km, and therefore the crust appears to have been thinned by at least ∼10 km since the Early Cretaceous. The relatively high MgO contents and Mg# of the Anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks, possibly coinciding with lower-crustal delamination, which would also account for the observed thinning.

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.

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.

Evidence suggests slab melting in arc magmas

Most recent geology textbooks state that subduction-related volcanism is due to the melting of the down-going lithosphere. However, for the last 30 years, few in the field have seriously believed that the subducting slab is the source of arc basalts. The accepted hypothesis involves melting of the mantle wedge above the slab via hydrous fluids produced during the transition of the subducting basalt from amphibolite to eclogite. The parental basalts differentiate primarily through crystal fractionation, magma mixing, and differentiation at the Mohorovicic discontinuity into andesites and dacites as they ascend; the basalts are too dense to rise through the lower continental crust. This explains the relative abundance of differentiated rocks in arcs.

Isotope and trace element evidence for three component mixing in the genesis of the North Luzon arc lavas (Philippines)

Post-3Ma volcanics from the N Luzon arc exhibit systematic variations in 87Sr/86Sr (0.70327–0.70610), 143Nd/144Nd (0.51302–0.51229) and 208Pb*/206Pb* (0.981–1.035) along the arc over a distance of about 500 km. Sediments from the South China Sea west of the Manila Trench also exhibit striking latitudinal variations in radiogenic isotope ratios, and much of the isotopic range in the volcanics is attributed to variations in the sediment added to the mantle wedge during subduction. However, Pb-Pb isotope plots reveal that prior to subduction, the mantle end-member had high Δ8/4, and to a lesser extent high Δ7/4, similar to that in MORB from the Indian Ocean and the Philippine Sea Plate. Th isotope data on selected Holocene lavas indicate a source with unusually high Th/U ratios (4.5–5.5). Combined trace element and isotope data require that three end-members were implicated in the genesis of the N Luzon lavas: (1) a mantle wedge end-member with a Dupal-type Pb isotope signature, (2) a high LIL/HFS ‘subduction component’ interpreted to be a slab-derived hydrous fluid, and (3) an isotopically enriched end-member which reflects bulk addition (<5%) of subducted S China Sea terrigenous sediment. The 87Sr/86Sr ratios in the volcanics show a restricted range compared with that in the sediments, and this contrasts with 143Nd/144Nd and 208Pb*/206Pb*, both of which have similar ranges in the volcanics and sediments. Such differences imply that whereas the isotope ratios of Nd, Pb and Th are dominated by the component from subducted sediment, those of Sr reflect a larger relative contribution from the slab-derived fluid.

Lithium isotope evidence for light element decoupling in the Panama subarc mantle

The systematics of fluid-mobile trace elements in arc lavas from Panama, relative to their Li isotopic compositions, provide unique evidence for the fertilization and subsequent differential extraction of mobile species from the subarc mantle. Calc-alkaline lavas that crystallized between 20 and 5 Ma (Old Group) that possess δ 7 Li as high as +11.2 have low B/Be. Otherwise identical (and similarly old) calcalkaline lavas with high B/Be (to 23), have mid-ocean ridge basalt (MORB) like δ 7 Li (+4.7 to +5.6). Adakite lavas (<3 Ma; Young Group) possess δ 7 Li from +1.4 to +4.2 and have consistently lower B/Be than Old Group lavas, consistent with derivation from melting of a devolatilized MORB slab. If Li and B had comparable fluid mobility in the subarc mantle, then slab fluids would carry both high B concentrations and elevated δ 7 Li signatures into arc sources, and samples with the highest δ 7 Li would also have the highest B/Be. Our data suggest that although both Li and B are initially derived from the slab, older δ 7 Li signatures may be preserved in the mantle beneath arcs. As a result, regions of the lithospheric mantle will develop Li isotope signatures that are heavier than typical MORB mantle.