Display omitted
•Shirvaneh- Meiham intrusive bodies were injected at 154 to 146 Ma ago.•Sequential injection of mafic magma made a wide range of magma mixing.•Sr-Nd isotope ratios indicated that ...source magma originated from the mantle.•Extensional tectonic regime was responsible for the sequential injection of magma.•The result shows continental crust getting thicker due to the Juvenile magma injection.
Intrusive bodies with a wide-ranging geology are distributed in the central part of the Sanandaj-Sirjan zone (C–SaSZ) in southern Ghorveh, western Iran. The zircon U–Pb ages were dated at 154–146 Ma, and were characterized by low 87Sr/87Sr(i) ratios and high 143Nd/144Nd(i) ratios that ranged from 0.7035 to 0.7061 and 0.51245 to 0.51271, respectively (positive εNd(t) values). Magma differentiation and mixing formed different magmatic rocks that range widely from gabbro to granite in the Meiham–Shirvaneh area in southern Ghorveh. However, there was no clear evidence to support the slab role for the enrichment of large-ion lithophile elements (LILEs) and light rare–earth elements (LREEs) in this area. Meanwhile, there is a possibility that some ocean island basalt-like melt, which has been reported in the Ghorveh area for the same period, was involved in the minor enrichment of LILEs and LREEs. The high development of the released depleted mantle melts, with a minor role played by crustal components, accelerated the thinning of the continental crust in an extensional regime such as continental rift in the C–SaSZ during the Jurassic. This rift was responsible for the sequential injection of mafic magma with a wide range of magma mixing and mingling in southern Ghorveh during the Late Jurassic. As a result, we observed a wide range of intrusive rocks from gabbro to granite with different compositions, which included some clear mixing and hybrid textures. This finding demonstrated how the released mantle melt formed a thicker crust in western Iran.
We present a dataset for boron and other trace element contents obtained from samples from 13 volcanoes distributed along the Quaternary volcanic front of the Southern Volcanic Zone (SVZ) of the ...Chilean Andes. The dataset shows constraints on the nature of slab-derived component to mantle source. Analyzed samples show large negative Nb and Ta anomalies, and enrichment of alkaline earth elements and Pb, which are features of typical island arc volcanic rocks. Boron contents of SVZ volcanic rocks range 2.3-125.5 ppm, exhibiting marked enrichment relative to N-MORB and OIB. Both the boron contents and B/Nb ratios of the volcanic rocks increase from the southern SVZ (SSVZ) to central SVZ (CSVZ). Fluid mobile/immobile element ratios (B/Nb, Ba/Nb, Pb/Nb, and K/Nb) are used to examine slab-derived component to mantle source. Trace element compositions of altered oceanic crust (AOC)-derived fluid, sediment-derived fluid, and sediment melt are modeled. Mantle sources of volcanic rocks in CSVZ with high B/Nb ratios were contaminated by both AOC-derived and sediment-derived fluids. In contrast, mantle sources of volcanic rocks in SSVZ with a low B/Nb ratio were contaminated with ca. 3 wt% melt of subducted sediment, which had suffered from loss of boron during progressive devolatilization before melting.
This paper newly reports results of LA-ICPMS U-Pb dating for 282 zircon crystals separated from a Middle Miocene adakite in Cerro Pampa, southern Argentine Patagonia. With the exception of one spot ...age, 174 of the U-Pb concordia ages are markedly older (>94 Ma) than the cooling ages of the adakite magma (ca. 12 Ma). The presence of numerous exotic zircon crystals indicates that the adakitic magma carries up information related to the crustal components during its ascent through the entire crust underneath Cerro Pampa. The obtained concordia ages of exotic zircons, 94-1335 Ma, are divisible into five groups having distinctive peaks on a population diagram. The first (94-125 Ma) and second age groups (125-145 Ma) correspond to the age of plutonic activities that formed the main body of the South Patagonian Batholith. The third to fifth groups respectively correspond to activities of the El Qumado-Ibañez volcanic complex (145-170 Ma), plutonic rocks scarcely exposed in Central Patagonia (170-200 Ma), and the Eastern Andean metamorphic complex of Late Paleozoic to Early Mesozoic ages (200-380 Ma). Our data suggest that the crust underneath Cerro Pampa was formed mostly after 380 Ma, the majority forming during the Early Cretaceous to Middle Jurassic. The processes of crustal development ceased for ca. 80 m.y. until the activity of the Cerro Pampa adakite in ca. 12 Ma. In contrast to the existence of numerous Archaean-Palaeoproterozoic exotic zircons in Mesozoic plutonic rocks distributed in Andean Cordillera at around 46°S, no evidence was found for Archaean-Paleoproterozoic crust on the Cerro Pampa region at 48°S. This evidence suggests that two crusts must have aggregated along a boundary between 46°S and 48°S with the continental margin of Gondwana during Late Paleozoic times, as part of the amalgamation of Pangea.
We estimated metamorphic conditions for the ∼
6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, ...changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge.
Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from ∼
230 °C in the volcanic unit to ∼
380 °C at the bottom of the gabbro unit, systematically ∼
200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration.
Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142).
IODP Hole 1256D penetrates a complete section of upper oceanic crust spread from the East Pacific Rise at a full rate of up to 220 mm/yr. Here we present U-Pb zircon data from Hole 1256D gabbroic ...rocks using isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and secondary ion mass spectrometry (SIMS). Three gabbro samples yielded Th-corrected weighted mean 206Pb/238U SIMS dates of 15.03 ± 0.14 Ma, 15.13 ± 0.35 Ma, and 15.23 ± 0.12 Ma. An ID-TIMS Th-corrected weighted mean 206Pb/238U date of 15.191 ± 0.040 Ma from the first of the three samples provides verification and refinement of the timing of magmatism. The U-Pb dates agree with the predicted age of the crust based on marine magnetic anomalies, suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis. Zircons from two of the studied samples have anomalously high Th/U ratios, likely reflecting oxidation of U prior to or during zircon crystallization. The elevated ƒO2 of late stage melts in the axial melt lens may be related to progressive crystallization, assimilation of hydrothermally altered sheeted dikes and gabbros, and/or contamination of the melt lens by seawater derived saline brines.
•TIMS and SIMS zircon U-Pb dates from in situ Miocene-age East Pacific Rise spread gabbros indicate axial melt emplacement.•In the context of global data, elevated Th/U values are consistent with oxidation of the axial melt lens.•The analyses support previous conclusions from petrologic and magnetic analyses of the Hole 1256D core.
The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly ...peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065–0.7085) and lower εNd(t) (−7.70 to −4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and −0.53 to −5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and −3.75 to −6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.
In the high-grade Cretaceous Sanbagawa high-pressure (HP) metamorphic belt, our new 1:5000 scale mapping of eclogitic mafic-ultramafic bodies and their surrounding epidote-amphibolite-facies schists ...has revealed a duplex structure formed by the subduction of the Izanagi-Pacific oceanic plate. Lithologies of the two largest mafic-ultramafic bodies in the Sanbagawa belt, the Iratsu eclogite and the Higashi-Akaishi peridotite, strike WNW-ESE and dip N; the upper boundary with the surrounding schist is a normal fault, whereas the lower boundary is a thrust. The Iratsu body is subdivided into at least two tectonic units; the unit boundary is subparallel to a lithological boundary. Protoliths of the upper unit are gabbro, basalt, minor quartz rock, and pelite, and those of the lower unit are pyroxenite, gabbro, basalt, chert, and marble, in ascending order. The lower unit is characterized by layers of alternating eclogitic metagabbro and pyroxenite. The layers are extensive at the bottom of the Iratsu eclogite, and transient toward the Higashi-Akaishi body. Eclogitefacies metapsammite is intercalated between the Iratsu and Higashi-Akaishi bodies. Our mapping has revealed the following: (1) a duplex structure of the mafic-ultramafic bodies indicating their accretionary complex origin; (2) reconstructed oceanic plate stratigraphy in ascending order of peridotite, gabbro, basalt, limestone, minor chert, and pelite, suggesting that different parts of the protolith were derived from a mid-oceanic topographic high, an oceanic island or plateau, and an overlying trench turbidite; and (3) a change in the convergent motion of the oceanic plate from NW to NE during the accretion of the large oceanic island or plateau.
The Late Miocene–Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile ...Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study, sensitive high mass‐resolution ion microprobe U–Pb and fission‐track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9 ± 0.4 and 5.6 ± 0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U–Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2 ± 0.2 Ma. These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid‐oceanic Ridge must have been emplaced during the 6 Ma event.
PDF
