Partial Melting Of Basaltic Crust Research Articles

Geochemical Signatures of Felsic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Greenstone Belts

Abstract Felsic volcanic rocks are abundant in ancient greenstone belts and important host rocks for volcanogenic massive sulfide (VMS) deposits. About half of all VMS deposits are hosted by dacite or rhyolite, an association that reflects anomalous heat flow during rifting, partial melting of basaltic crust, and fractional crystallization in high-level magma chambers. For over 30 years, geochemical signatures of these rocks (e.g., F classification of Archean rhyolites) have been widely used to identify possible hosts for VMS deposits in ancient greenstone belts. However, comparisons with modern oceanic settings have been limited, owing to a lack of samples of felsic volcanic rocks from the sea floor. This is changing with increasing exploration of the oceans. In this study, we have compiled high-quality geochemical analyses of more than 2,200 unique samples of submarine felsic volcanic rocks (>60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hot-spot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical diversity spanning the full range of compositions of rhyolites found in ancient greenstone belts. This diversity is interpreted to reflect variations in crustal thickness, the presence or absence of slab-derived fluids (dry melting versus wet melting), and mantle anomalies. Highly variable melting conditions are thought to be related to short-lived microplate domains, such as those caused by diffuse spreading and multiple overlapping spreading centers. Systematic differences in the compositions of felsic volcanic rocks in the modern oceanic settings are revealed by a combination of principal components analysis, unsupervised hierarchical clustering, and supervised random forest classification of the compiled data. Dacites and rhyolites from midocean ridge settings have moderately depleted mantle signatures, whereas rocks from ridge-hot-spot intersections and ocean islands reflect enriched mantle sources. Felsic volcanic rocks from arc-back-arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry (strong large ion lithophile element enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements [HREEs]). This contrasts with felsic volcanic rocks in Archean greenstone belts, which show high field strength element and HREE enrichment (so-called FIIIb-type) due to a less depleted mantle, a lack of wet melting, and variable crustal contamination. The differences between modern and ancient volcanic rocks are interpreted to reflect the lower mantle temperatures, thinner crust, and subduction-related processes in present-day settings. We suggest that the abundance of FIIIb-type felsic volcanic rocks in Archean greenstone belts is related to buoyant microplate domains with thickened oceanic crust that were better preserved on emerging Archean cratons, whereas in post-Archean tectonic settings most of these rocks are subducted.

Late Mesozoic magmatism and tectonic significance of the Kelihe Area in the northern Great Xing'an Range

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogenic belt in the world and its formation involved a complicated process of arc–continent collision. Large‐scale Mesozoic volcanic activity in the Great Xing'an Range (GXAR) in northeastern China, which constitutes part of the eastern CAOB, has received much recent research attention. However, the petrogenesis and formation mechanism of the late Mesozoic igneous rocks remain enigmatic. This paper investigates the geochronology, petrogenesis, and formation mechanism of late Mesozoic magmatic rocks in the Kelihe area of the GXAR based on zircon U–Pb dating, petrology, and geochemistry. Late Mesozoic magmatism in the Kelihe area occurred during the Late Jurassic (165–155 Ma) and Early Cretaceous (130–120 Ma). These igneous rocks generated by this magmatism are characterized by high SiO2 (69.97–75.49 wt%) and low MgO (0.11–0.68 wt%), Cr (10–30 ppm), Co (0.2–2.6 ppm), and Ni (0.5–1.6 ppm) contents. Trace elements show enrichment in Rb, Th, U, Zr, and K, and depletion in Ba, P, Nb, and Ti. These features indicate that the rocks were derived from partial melting of the crust. Furthermore, the rhyolites have high Na2O (5.02–5.44 wt%) and K2O (3.94–4.88 wt%) contents, similar to those of melts formed by the partial melting of basaltic crust. Combining the present results with the findings of previous studies, it is inferred that the Mesozoic igneous rocks formed in a post‐collisional extensional setting related to lithospheric thinning and asthenospheric upwelling after the closure of the Mongol–Okhotsk Ocean.

Neoarchaean Felsic Volcanic Rocks in Tracing Evolution of Arcs: An Insight from Geochemical Data of the Gadag Schist Belt, Western Dharwar Craton

Abstract Quartz porphyry of rhyolitic composition though a volumetrically minor is an important component of metavolcanic successions in the Gadag schist belt in the western Dharwar craton. The quartz porphyries are calc-alkaline, enriched in light rare earth elements (Lan/Ybn ~ 10-20) and with a strong negative Eu anomaly (Eu/Eu*N = 0.45-0.58). These features, coupled with high K2O/Na2O (0.8 to 1.46), low Nb/Th (<1), and Lan/Smn ratios of 4.2 to 6.0, suggest that rhyolitic magma formed from melt generated by partial melting of basaltic crust, subsequently modified by crustal assimilation and crystal fractionation. The geological setting of the quartz porphyries and the geochemical relationships suggest magmatism at a convergent plate tectonic setting. The zircon U-Pb age of 2588.0 ± 2.4 Ma for the quartz porphyry, together with an age of 2732 Ma for similar volcanic rocks in the Chitradurga schist belt further south, and 2614 Ma for felsic volcanics from the Shimoga schist belt, point to an extended period with episodic arc magmatism in the western Dharwar craton of at least ~144myr.

Geochronological and geochemical constraints on the petrogenesis of the 2.6–2.5 Ga amphibolites, low- and high-Al TTGs in the Wangwushan area, southern North China Craton: Implications for the Neoarchean crustal evolution

The crustal evolutionary processes of the North China Craton (NCC) during the late Neoarchean (ca. 2.6–2.5 Ga) are still under large debate. The systematic investigations of widespread ca. 2.6–2.5 Ga meta-volcanic basalts and TTG assemblages can provide a better understanding of tectonic evolution at this period. The Wangwushan Neoarchean amphibolites, high- and low-Al TTGs in southern NCC were formed at 2.57–2.52 Ga. The amphibolites are tholeiitic in composition. They have whole-rock εNd(t) values of −2.8–+4.0 (TDM1 = 3.7–2.6 Ga), and zircon εHf(t) values of +1.2–+7.2 (TDM1 = 2.81–2.57 Ga), indicative of a depleted mantle source with involvement of older crustal materials. Their compositional spectrum accords with those in island-arc field, or straddles the boundary between MORB and island-arc fields, suggesting significant contributions from subduction zone-derived fluids during magma formation. The high-Al TTGs contain high Mg#, Cr and Ni contents, and (La/Yb)N and Sr/Y ratios, together with positive whole-rock εNd(t) values (−0.6–+5.2) (TDM2 = 2.95–2.48 Ga), indicating that they derived from partial melting of a subducted oceanic slab with older crustal materials. Their geochemical features are corresponding to ‘slab-melt’ identification criterion in an arc setting, and illustrate a deep source (>45 km) with garnet amphibolite in the residue without rutile. The low-Al TTGs show low Mg# and (La/Yb)N ratios, negative Eu anomalies, relatively flat HREE (and Y) patterns. Their zircon εHf(t) (+2.2–+8.8 and +1.3–+6.9) and whole-rock εNd(t) values (ca. +2.23) are positive. All the evidences support that the low-Al TTGs derived from a low degree partial melting of basaltic crust with the residue lack of garnet, consistent with a shallower depth (about <30 km). It is likely that the underplated basaltic or high-Al magmas gave rise to re-melting newly formed lower crust to form the low-Al TTGs. Combined with ca. 2.52 Ga diorites and high-K granites in this area, which were formed in a subduction process and post-collisional setting, respectively, we propose that all of these igneous rocks record multi-stage processes in a convergent plate margin.

Magmatic and tectonic emplacement of the Pukaskwa batholith, Superior Province, Ontario, CanadaThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

The Neoarchean Pukaskwa batholith consists of pre-, syn-, and post-tectonic phases emplaced over an interval of 50 million years. Pre-tectonic phases are broadly synvolcanic and have a high-Al tonalite–trondhjemite–granodiorite (TTG) affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths. Minor syn-tectonic phases slightly post-date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic (mantle) source component. Magmatic emplacement of pre- and syn-tectonic phases occurred in the midcrust at paleopressures of 550–600 MPa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development. Subsequent to emplacement of the syntectonic phases, and likely at approximately 2680 Ma, the Pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral transpressive deformation. The driving force for vertical tectonism is interpreted to be density inversion (Rayleigh–Taylor-type instabilities) involving denser greenstone belts and underlying felsic plutonic crust. The trigger for initiation of this process is interpreted to be an abrupt change in the rheology of the midcrust attributed to introduction of heat from the mantle attendant with slab breakoff or lithospheric delamination following the cessation of subduction. This process also led to partial melting of the intermediate to felsic midcrust generating post-tectonic granitic phases at approximately 2667 Ma. We propose that late density inversion-driven vertical tectonics is an inevitable consequence of horizontal (plate) tectonic processes associated with greenstone belt development within the Superior Province.

Magma genesis beneath Northeast Japan arc: A new perspective on subduction zone magmatism

It is being accepted that earthquakes in subducting slab are caused by dehydration reactions of hydrous minerals. In the context of this “dehydration embrittlement” hypothesis, we propose a new model to explain key features of subduction zone magmatism on the basis of hydrous phase relations in peridotite and basaltic systems determined by thermodynamic calculations and seismic structures of Northeast Japan arc revealed by latest seismic studies. The model predicts that partial melting of basaltic crust in the subducting slab is an inevitable consequence of subduction of hydrated oceanic lithosphere. Aqueous fluids released from the subducting slab also cause partial melting widely in mantle wedge from just above the subducting slab to just below overlying crust at volcanic front. Hydrous minerals in the mantle wedge are stable only in shallow (< 120 km) areas, and are absent in the layer that is dragged into deep mantle by the subducting slab. The position of volcanic front is not restricted by dehydration reactions in the subducting slab but is controlled by dynamics of mantle wedge flow, which governs the thermal structure and partial melting regime in the mantle wedge.

Late Paleozoic adakites and Nb‐enriched basalts from northern Xinjiang, northwest China: Evidence for the southward subduction of the Paleo‐Asian Oceanic Plate

Abstract Volcanic rocks consisting of adakite and Nb‐enriched basalt are found in the early Devonian Tuoranggekuduke Group in the northern margin of the Kazakhstan‐Junggar Plate, northern Xinjiang, northwest China. The geochemical characteristics of the andesitic and dacitic rocks in this area resemble that of adakites. The relatively high Al2O3, Na2O and MgO content and Mg♯ values indicate that the adakites were generated in relation to oceanic slab subduction rather than the partial melting of basaltic crust. A slightly higher SrI and a lower ɛNd(t = 375 Ma) compared to adakites of mid‐oceanic ridge basalt (MORB) imply that slab sediments were incorporated into these adakites during slab melting. The Nb‐enriched basalt lavas, which are intercalated in adakite lava suite, are silica saturated and are distinguished from the typical arc basalts by their higher Nb and Ti content (high field strength element enrichment). They are derived from the partial melting of the slab melt‐metasomatized mantle wedge peridotite. Apparently, positive Sr anomalies and a slightly higher heavy rare earth element content in these adakites compared to their Cenozoic counterparts indicate that the geothermal gradient in the Paleo‐Asian Oceanic subduction zone and the depth of the Paleo‐Asian Oceanic slab melting are between those of their Archean and Cenozoic counterparts. The distribution of the adakites and Nb‐enriched basalts in the northern margin of the Kazakhstan‐Junggar Plate, northern Xinjiang, indicates that the Paleo‐Asian Oceanic Plate subducted southward beneath the Kazakhstan‐Junggar Plate in the early Devonian period.

Basaltic magma from the crust is not a free option

Supposing that it is the business of section officers to stir the pot from time to time, I have elected to introduce myself (as President‐Elect of the Volcanology, Geochemistry, and Petrology section) by way of this small polemic on a matter of general interest, the origin of mafic magma. My hope is to identify a piece of folly so that it will go away. Knowing, however, the Gresham‐Murphy law of information that bad ideas outrun the good, my expectations are somewhat lower than my hopes.In a recent abstract about the perturbed geochemistry of mafic crustal rocks, the usual options were invoked: enriched mantle source, crustal contamination of basaltic magma, and, in addition, “ … by partial melting of basaltic crust in the lower crust or uppermost mantle.” This last hypothesis appears with some frequency in the literature. Accordingly, I refrain from pointing the finger at the amiably misguided perpetrators by citing references to any of them.

Geotherms, evolution of the lithosphere and plate tectonics

For the purpose of calculating Precambrian geotherms, modern “average” geotherms need to be defined. Curves calculated from estimated plate sections are close to those of Clark and Ringwood (1964). Thicknesses of lithospheric plates at any time in the past can be estimated from “fossil” geotherms. Because subduction of oceanic plates depends upon their density, average densities of Precambrian plates should place important constraints on the beginning of subduction. This method, however, only indicates that in all probability subduction did not begin before 1000 Ma ago. Geological data suggest a date of about 800 or 900 Ma ago. Formation of Archean continents would have proceeded from partial melting of basaltic crust, and the end of that phase of earth's evolution would be a thermally controlled feedback mechanism lowering geothermal gradients in sub-continental mantle. The beginning of subduction was also thermally controlled, and probably accompanied the increasing stability of eclogite in the upper mantle.