Melting Of Basaltic Lower Crust Research Articles

The early–middle Silurian Delite monzogranite and quartz syenite, East Kunlun Orogenic Belt, NW China: petrogenesis and implications for tectonic evolution of the Proto‑Tethys

The tectonic evolution of the Proto‑Tethys in the East Kunlun Orogenic Belt (EKOB) is controversial. Herein, new petrological, whole-rock geochemical, zircon U–Pb geochronological and zircon Lu–Hf isotopic data for early Paleozoic intrusive rocks in the Delite region are reported. LA-ICP-MS zircon U–Pb isotopic data show that the Delite monzogranite and quartz syenite were formed at 437.0 ± 2.9 Ma and 424.8 ± 2.2 Ma, respectively. The Delite monzogranite contains high SiO2, Al2O3 and K2O and A/CNK ratios, but low MgO and Mg# values (33–38). This study considers that the monzogranite is a typical S-type granite and originated from partial melting of ancient crustal clastic material (metagreywacke) in a syn-collision setting. The quartz syenite is K-rich and Mg-poor, with low Mg# values (36–41), exhibiting geochemical signatures of an alkalic syenite derived from partial melting of basaltic lower crust in a post-collision setting. Combining with the previous work, we suggest that in the middle part of the EKOB the Proto-Tethys closed at ca 437 Ma with a post-collisional setting after ca 427 Ma. KEY POINTS Zircons LA-ICP-MS U–Pb isotopic data indicate that the Delite monzogranite and quartz syenite were formed at 437.0 ± 2.9 Ma and 424.8 ± 2.2 Ma, respectively. The Delite monzogranite was formed in a syn-collision setting. The Delite quartz syenite was related to the post-collision setting. It is suggested that the Proto-Tethys closed at ca 437 Ma in the middle part of the EKOB, and the post-collisional setting was after ca 427 Ma in the EKOB.

Petrogenesis and magmatic evolution of the intermediate–felsic Early Cretaceous Shizhuzi magmatic complex on Liaodong Peninsula, NE China

The destruction of the North China Craton (NCC) culminated during the Early Cretaceous, inducing widespread magmatism and related mineralization in the eastern China. The Shizhuzi magmatic complex (SMC) from the Liaodong Peninsula is typical products of this event, thus could provide important insights on the destruction process. Here, we present petrological, geochronological, and elemental and isotopic geochemical data for the SMC rocks in order to reveal their magma sources, petrogenesis and the possible geodynamic process. The complex comprises quartz diorite, granodiorite, and granite, accompanied with Au, Cu and Mo mineral deposits. Zircon UPb dating shows that the different rocks were coeval with the formation ages of 129–126 Ma, coincident with the associated mineralization. However, field relations, whole-rock major and trace element contents and isotopic compositions indicate different origin for the quartz diorites and the coeval granodiorites and granites. The quartz diorites are calc-alkaline rocks with high MgO, Mg# and low SiO2. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs) with significant negative Nb, Ta and Ti anomalies. These geochemical data and the highly enriched zircon Hf isotopes suggest that they were derived from partial melting of enriched subcontinental lithospheric mantle. The granodiorites and granites from the SMC have high SiO2 and low MgO contents, suggesting that they were derived from crustal sources. These rocks are I-type rocks with high-K calc-alkaline features. Their primary magma originated from the partial melting of basaltic lower crust and experienced fractional crystallization. The ascending mantle-derived magma provided heat for the melting of the crustal rocks. Based on regional geology, the SMC was generated in the extensional setting related to the roll-back of the subducted Paleo-Pacific plate. The concurrent mantle and crustal melting in the formation of the SMC revealed the loss of stability of the cratonic lithosphere, related to the destruction of the NCC during Early Cretaceous. Besides, the emplacement of SMC might also contribute to the formation of contemporaneous Au, Mo, and Cu deposits.

Geochemistry of Early Cretaceous volcanic rocks in the Northeastern Great Xing’an Range, northeast China and implication for geodynamic setting

ABSTRACTThe mechanism that triggered large-scale Late Mesozoic magmatism in the northeastern Great Xing’an Range (NE GXAR) is strongly controversial. In this paper, we present whole rock geochemistry and zircon trace element, U-Pb and Hf isotopic data on the volcanic rocks in the Longjiang and Guanghua formations in the northeastern Xing’an Block. Zircons with ages of 120–119 Ma indicate that these volcanic rocks were formed in the Early Cretaceous. Combined with previous data, it is clear that volcanic rocks in the NE GXAR erupted between 128 and 108 Ma. The andesite samples of the Longjiang Formation show high contents of Al2O3, CaO, and MgO, significant negative Nb, Ta, and Ti anomalies; εHf (t) values of zircons from the andesite sample vary from +4.13 to +7.67, indicating an enriched mantle source. The rhyolites of the Guanghua Formation show high SiO2 and K2O concentrations, low P2O5, MgO, Cr, and Ni contents and Mg# values. The positive εHf (t) values (+5.72 to +10.58) with two-stage Hf model ages ranging from 939 to 701 Ma indicate that the rhyolites are derived from the partial melting of basaltic lower crust. Combined with the regional geological evolution, we conclude that the generation of the Early Cretaceous volcanic rocks in the NE GXAR might be triggered by the dehydration, disintegration, and foundering of the Mongol-Okhotsk Oceanic flat-slab and the subsequent upwelling of the asthenosphere.

The Jurassic Yeba Formation in the Gangdese arc of S. Tibet: implications for upper plate extension in the Lhasa terrane

ABSTRACTThe formation of the Jurassic Xiongcun porphyry copper deposit, within the Gangdese magmatic belt of southern Tibet, took place coincident with subduction of the Neotethyan oceanic lithosphere. Although the Yeba Formation (coeval and co-magmatic intrusions and volcanic rocks) in the eastern Gangdese magmatic belt formed at the same time as the Xiongcun deposit, the Yeba Formation is devoid of subduction-related porphyry Cu deposits. Here, we report detrital zircon U–Pb ages for sedimentary rocks of the Yeba and Bima Formations along with a summary of all published whole-rock geochemical data and zircon epsilon Hf isotopes for the volcanic rocks in the Yeba Formation. We use these data to re-evaluate the petrogenesis, magmatic source, and tectonic setting of the volcanic rocks. Detrital zircon ages from the sedimentary rocks reveal that the volcanic rocks of the Yeba and Bima Formations are the main source for their sedimentary interlayers. In addition, the Yeba sedimentary layers contain inherited zircons sourced from older country rocks whereas the Bima Formation is devoid of ancient zircons. Geochemically, the Yeba volcanic rocks resemble a bimodal volcanic suite. Geochemical data reveal that the Yeba volcanic rocks formed in a continental arc setting, while the mafic ones were derived from the partial melting of the mantle wedge above the subducting slab rather than from the melting of the descending oceanic slab. In addition, the mafic ensemble formed in a back-arc setting, indicative of subduction-related extension in the Lhasa terrane. In contrast, the felsic ensemble may originate from the melting of basaltic lower crust. Thus, the magma source and tectonic setting for the Yeba Formation are distinct from typical subduction-related Cu-hosted porphyry and volcanic rocks in a compressional setting.

The Paleozoic tectonic evolution and metallogenesis of the northern margin of East Junggar, Central Asia Orogenic Belt: Geochronological and geochemical constraints from igneous rocks of the Qiaoxiahala Fe-Cu deposit

The East Junggar terrane (NW China) is an important constituent of the Central Asian Orogenic Belt (CAOB). From the Devonian to Permian, regional magmatism evolved from mainly calc-alkaline (I-type) to alkaline (A-type). The Qiaoxiahala Fe-Cu deposit, located in the Late Paleozoic Dulate island arc (northern margin of the East Junggar), is hosted in the volcanic rocks of the Middle Devonian Beitashan Formation. Two magmatic stages were identified in the deposit, the Qiaoxiahala diorite porphyry (380±4.0Ma) and a younger aplite (331±3.1Ma).The (high-K) calc-alkaline Beitashan Formation basaltic rocks are characterized by LILE and LREE enrichments and HFSE depletions, pointing to a subduction-related affinity. The high Mg# (42–75), elevated Ce/Th and Ba/Th, depleted Nb, positive εNd(t) (6.6), low (87Sr/86Sr)i (0.7037) and MORB-like Pb isotope characters all suggest an origin involving partial melting of a MORB-like depleted mantle wedge (metasomatized by slab-derived fluids) with little evidence of crustal contamination.The calc-alkaline (I-type) diorite porphyry, characterized by LILE and LREE enrichments and HFSE depletions, may have formed from fractional crystallization of the basaltic rocks, with its parental magma derived from the same depleted mantle wedge. The negative εHf(t) (−8.26), Hf model age (TDMC) of 1406Ma and the presence of inherited zircons (ca. 470 and 506Ma) indicate that the diorite has assimilated older crustal material.The alkaline, metaluminous (A-type) aplite is characterized by HFSE enrichment and depletions in Sr, P and Ti, distinct from the basaltic rocks and diorite porphyry at Qiaoxiahala. The low Mg# (35–38), positive Zr and Hf, positive εHf(t) (4.77–9.75) and εNd(t) (6.85–6.86) and low T2DM (538–520Ma) suggest a juvenile lower crustal source due to partial melting of basaltic lower crust as a result of underplating of mantle-derived melts and accompanied by magma mixing.The tectonic evolution of the Paleozoic East Junggar was associated with the initiation of subduction of the Paleo-Asian Ocean, with the formation of an island arc during the Early to Middle Devonian and generation of the coeval arc-related Fe-Cu (-Au) and porphyry Cu mineralization. Collision between the Chinese Altay and East Junggar happened in the Late Devonian with Early Carboniferous post-collisional tectonics likely explaining the synchronous Cu-Mo mineralization, while the late-Early to Late Carboniferous post-orogenic event ended without related mineralization in Qiaoxiahala.

Continental geochemical signatures in dacites from Iceland and implications for models of early Archaean crust formation

Whether early Archaean felsic crust was formed by processes related to plate subduction or melting of thick basaltic plateaus is vividly debated. Ultimately, the discussion hinges on the question of how Archaean felsic crust has obtained its distinct chemical traits. Here we report chemical and isotopic data for a suite of Cenozoic felsic volcanic rocks from Iceland. The samples exhibit the key-chemical characteristics of early Archaean felsic continental crust such as calc-alkaline composition, strong enrichment in Na relative to K, high Pb/Ce, La/Nb, and Ta/Nb ratios. Involvement of pre-existing continental lithosphere in the petrogenesis of the samples can be excluded, because their 207Pb/ 204Pb and 206Pb/ 204Pb ratios plot well within the range of Iceland basalts. Model calculations suggest that the chemical characteristics were produced by high-pressure partial melting of basaltic lower crust followed by fractional crystallisation of amphibole, plagioclase, and ilmenite. These findings demonstrate that plate subduction and melting of subduction-modified mantle or lithosphere are not necessarily required to produce the key-chemical signatures of continental crust. Hence, the calc-alkaline dacites provide intriguing support for early Archaean continental crust formation by melting of thick mafic plateaus.

Silicic magmas of Protector Shoal, South Sandwich arc: indicators of generation of primitive continental crust in an island arc

Protector Shoal, the northernmost and most silicic volcano of the South Sandwich arc, erupted dacite–rhyolite pumice in 1962. We report geochemical data for a new suite of samples dredged from the volcano. Geochemically, the dredge and 1962 samples form four distinct magma groups that cannot have been related to each other, and are unlikely to have been related to a single basaltic parent, by fractional crystallization. Instead, the silicic rocks are more likely to have been generated by partial melting of basaltic lower crust within the arc. Trace element and Sr–Nd isotope data indicate that the silicic volcanics have compositions that are more similar to the volcanic arc than the oceanic basement formed at a back-arc spreading centre, and volcanic arc basalts are considered to be the likely source for the silicic magmas. The South Sandwich Islands are one of several intra-oceanic arcs (Tonga–Kermadec, Izu–Bonin) that have: (1) significant amounts of compositionally bimodal mafic–silicic volcanic products and (2) 6.0–6.5 km s−1P-wave velocity layers in their mid-crusts that have been imaged by wide-angle seismic surveys and interpreted as intermediate-silicic plutons. Geochemical and volume considerations indicate that both the silicic volcanics and plutonic layers were generated by partial melting of basaltic arc crust, representing an early stage in the fractionation of oceanic basalt to form continental crust.

Petrogenesis of the Mesozoic intrusive rocks in the Tongling area, Anhui Province, China and their constraint on geodynamic process

Petrology, element and isotopic geochemistry of the Mesozoic intrusive rocks in the Tongling area were systematically investigated in this study. The intrusive rocks can be divided into two groups, one contains shoshonitic rocks with SiO 2 ≤55%, the other consists mainly of high-potassic calc-alkaline rocks with SiO 2 >55%. The shoshonitic rocks (SiO 2 ≤55%) were generated by the fractional crystallization of the primary basaltic magma sourced from an enriched mantle, then the evolved basaltic magma likely experienced low-degree contamination with the lower crust materials when they ascended. On the other hand, although the intrusive rocks with SiO 2 >55% show most elemental geochemical characteristics similar to an adakite, such as high Na 2 O, Al 2 O 3 , Sr contents, high Sr/Y and La/Yb ratios, they have isotopic compositions much different from an adakite, such as low e Nd(t) (-9.16—16.55) and high ( 87 Sr/ 86 Sr) i (0.7068—0.7105), and some of them show relatively high Y and Yb contents than those of an adakite. We propose that the intrusive rocks with SiO 2 >55% were most probably produced by mixing of the mantle-derived basaltic magma and adakite-like magma derived from the melting of basaltic lower crust that was heated by the underplating mantle-derived shoshonitic magmas. The delamination of lower crust likely took place after or during the formation of these adakite-like rocks in the Tongling area.

Formation of tonalite from basaltic magma at the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge in the Philippine Sea, and growth of Izu-Ogasawara (Bonin)-Mariana arc crust

Tonalitic rocks dredged from the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge are classified as biotite–hornblende tonalites and hornblende tonalites. These rocks have radiometric ages of 37–38 Ma, indicating that felsic plutonic activity occurred during the early stages of Izu-Ogasawara (Bonin)-Mariana (IBM) arc volcanism. Therefore, this tonalite complex has great importance for understanding the initial processes of island arc and continental crust formation. These tonalitic rocks exhibit the following petrological and geochemical characteristics: (1) common lamellar twins and oscillatory zoning patterns in plagioclase phenocrysts throughout the compositional range; (2) hornblende tonalite shows parallel REE patterns and increasing total REE content with increasing SiO2, except for an increasingly strong negative Eu anomaly at higher SiO2 levels; and (3) isotopic composition remains constant over a wide silica variation. We compare this tonalite with younger tonalities of the same arc from the Tanzawa Complex (10–5 Ma), central Japan, considered to represent the lower–middle crust of the IBM arc, and find the following differences: (1) cumulate textures found in Tanzawa tonalites are not observed in samples from the Komahashi-Daini Seamount; and (2) Komahashi-Daini Seamount tonalites, unlike those from Tanzawa, exhibit linear variations of Zr and REEs vs. SiO2 plots. These data and other observations support the interpretation that tonalite in the Komahashi-Daini Seamount was produced by crystal fractionation from basaltic magma. We suggest that fractional crystallization operated during the early stage of oceanic island arc formation to produce tonalite, whereas tonalities in later stages formed largely by partial melting of basaltic lower crust, as represented by the tonalites in the Tanzawa Complex.

Intra-oceanic subduction systems: introduction

Abstract Intra-oceanic arcs are the simplest type of subduction systems in that they occur where overridding plates of subduction zones consist of oceanic rocks, contrasting with arcs built on continental margins. They comprise some 40% of the subduction margins of the Earth. The better-known examples include the Izu-Bonin-Mariana arc, the Tonga-Kermadec arc, the Vanuatu arc, the Solomon arc, the New Britain arc, the western part of the Aleutian arc, the South Sandwich arc and the Lesser Antilles arc. They are thought to represent the first stage in the generation of continental crust from oceanic materials. They are generally more inaccessible than continental arcs, but, for a variety of reasons, provide insights into processes in subduction zones that are impossible or difficult to glean from the better-studied continental arcs. Intra-oceanic arcs typically have a simpler crustal structure than arcs built on continental crust, although there are significant differences between examples. Geochemically, magmas erupted in intra-oceanic arcs are not contaminated by ancient sialic crust, and their compositions more accurately record partial melting processes in the mantle wedge. They are also the sites of generation of intermediate-silicic middle crust and volcanic rocks, probably representing the earliest stage of generation of andesitic continental crust by partial melting of basaltic lower crust. They are the best locations in which to study mantle flow in the vicinity of subducting slabs using both geophysical and geochemical methods. They are the sites of significant hydrothermal activity and metallogenesis. The fact that their hydrothermal discharges typically occur shallower in the ocean than those from mid-ocean ridge vents means that they have the potential for greater environmental impact.

The Petrogenesis of Felsic Calc-alkaline Magmas from the Southernmost Cascades, California: Origin by Partial Melting of Basaltic Lower Crust

The Petrogenesis of Felsic Calc-alkaline Magmas from the Southernmost Cascades, California: Origin by Partial Melting of Basaltic Lower Crust

The Petrogenesis of Felsic Calc-alkaline Magmas from the Southernmost Cascades, California: Origin by Partial Melting of Basaltic Lower Crust

The majority of felsic rocks from composite centers in the southernmost INTRODUCTION Cascades have geochemical and Sr, Nd and Pb isotopic ratios Despite the importance of felsic magmas in calc-alkaline that suggest derivation by partial melting of lower crust that is rock suites, as both the end of the compositional spectrum compositionally similar to calc-alkaline basalts observed in the region. of lavas and as a potential assimilant or mixing endOnly a few felsic rocks have d 18 O and Pb isotopic compositions member, most petrogenetic studies have focused on bathat indicate interaction with the upper crust. Mineralogical and saltic magmas. This stems from the fact that sparsely geochemical diVerences among the felsic magmas result primarily crystalline basaltic lavas are usually the most primitive from melting under variable f(H2O) and temperature conditions. lavas within a given suite of rocks, and therefore have Partial melting under low f(H2O) and high temperature conditions compositions that are most indicative of magma sources. leaves an amphibole-poor residuum, and produces magmas that In contrast, felsic rocks have high incompatible element have orthopyroxene as the most abundant ferromagnesian phenocryst, abundances and are often highly crystalline, and may relatively low silica contents, and straight rare earth element patterns. therefore have been extensively modified by fractional Partial melting under higher f(H2O) and lower temperature con- crystallization and assimilation fractional crystallization ditions leaves an amphibole-rich residuum, and produces magmas (AFC). This study focuses on sparsely crystalline felsic that have amphibole ‐ biotite phenocrysts, relatively high silica (SiO2 > 68 wt %) volcanic rocks erupted in the southcontents, and pronounced middle rare earth element depletions. These ernmost Cascades that demonstrate evidence for only a conclusions are consistent with published thermal models that suggest minor amount of diVerentiation. We use mineralogy, that reasonable volumes of basaltic magma emplaced beneath large major and trace element geochemistry, and isotopic compositions of felsic rocks erupted from composite cencomposite centers in the southernmost Cascades can serve as the ters to constrain their petrogenesis. These felsic rocks heat source for melting of the lower crust. Melting of the lower provide new insights into petrogenetic processes that crust under variable f(H2O) conditions is likely to result from generate felsic arc magmas.