This study presents geochronological and geochemical data from newly dated Permian granitic orthogneisses associated with the Eclogite-Gneiss unit (EGU) from the southernmost part of the Austroalpine nappe stack, exposed within the Pohorje Mountains (Slovenia). LA-ICP-MS zircon U–Pb ages of two samples of the augen-gneisses are 255 ± 2.2 Ma and 260 ± 0.81 Ma, which are interpreted as the age of magmatic crystallization of zircon. In contrast, all round zircons from leucogneisses give Cretaceous ages (89.3 ± 0.7 Ma and 90.8 ± 1.2 Ma), considered as the age of UHP/HP metamorphism. The round zircons overgrew older euhedral zircons of Permian and rare older ages tentatively indicating that these rocks are of latest Permian age, too. Zircon εHf(t) values of the four orthogneiss samples are between − 13.7 and − 1.7 with an initial 176Hf/177Hf ratio ranging from 0.282201 to 0.282562; T DM C is Proterozoic. The augen-gneisses show geochemical features, e.g. high (La/Lu)N ratios and strong negative Eu anomalies, of an evolved granitic magma derived from continental crust. The leucogneisses are more heterogeneously composed and are granitic to granodioritic in composition and associated with eclogites and ultramafic cumulates of oceanic affinity. We argue that the Permian granitic orthogneisses might be derived from partial melting of lower crust in a rift zone. We consider, therefore, that segment of the EGU is part of the distal Late Permian rift zone, which finally led to the opening of the Meliata Ocean during Middle Triassic times. If true, the new data also imply that the Permian stretched continental crust was potentially not much wider than ca. 100 km, was subducted and then rapidly exhumed during early Late Cretaceous times.
The Grobgneis complex, located in the eastern Austroalpine unit of the Eastern Alps, exposes large volumes of pre-Alpine porphyric metagranites, sometimes associated with small gabbroic bodies. To better understand tectonic setting of the metagranites, we carried out detailed geochronological and geochemical investigations on the major part of the porphyric metagranites. LA–ICP–MS zircon U–Pb dating of three metagranites sampled from the Grobgneis complex provides the first reliable evidence for large volumes of Permian plutonism within the pre-Alpine basement of the Lower Austroalpine units. Concordant zircons from three samples yield ages at 272.2 ± 1.2 Ma, 268.6 ± 2.3 Ma and 267.6 ± 2.9 Ma interpreted to date the emplacement of the granite suite. In combination with published ages for other Permian Alpine magmatic bodies, the new U–Pb ages provide evidence of a temporally restricted period of plutonism (“Grobgneis”) in the Raabalpen basement Complex during the Middle Permian. Comparing the investigated basement with that of the West Carpathian basement, we argue that widespread Permian granite magmatism occurred in the Lower Austroalpine units. They belong to the high-K calc-alkaline to shoshonitic S-type series on the base of geochemical data. Zircon Hf isotopic compositions of the Grobgneis metagranites show εHf(t) values of − 4.37 to − 0.6, with TDM2 model ages of 1.31–1.55 Ga, indicating that their protoliths were derived by the recycling of older continental crust. We suggest that the Permian granitic and gabbroic rocks are considered as rifted-related rocks in the Lower Austroalpine units and are contemporaneous with cover sediments.
Many metamorphosed basement complexes in the Alps are polymetamorphic and their origin and geological history may only be deciphered by detailed geochronology on the different members including oceanic elements like ophiolites, arc successions, and continental passive margin successions. Here we present a case study on the Lower Austroalpine Variegated Wechsel Gneiss Complex and the overlying low-grade metamorphosed Wechsel Phyllite Unit at the eastern margin of Alps. The Wechsel Gneiss Complexes are known to have been overprinted by Devonian metamorphism, and both units were affected by Late Cretaceous greenschist facies metamorphism. New U–Pb zircon ages reveal evidence for two stages of continental arc-like magmatism at 500–520 Ma and 550–570 Ma in the Variegated Wechsel Gneiss Complex. An age of ca. 510 Ma of detrital zircons in metasedimentary rocks also constrain the maximum age of metasedimentary rocks, which is younger than Middle Cambrian. The overlying Wechsel Phyllite Unit is younger than 450 Ma (Late Ordovician) and seems to have formed by denudation of the underlying Variegated Wechsel Gneiss Complex. We speculate on potential relationships of the continental arc-type magmatism of the Variegated Wechsel Gneiss Complex and potential oceanic lithosphere (Speik complex) of Prototethyan affinity, which is also preserved in the Austroalpine nappe complex. The abundant, nearly uniform 2.1 Ga- and ca. 2.5 Ma-age signature of detrital zircons in metasediments (paragneiss, quartzite) of the Variegated Wechsel Gneiss Complex calls for Lower Proterozoic continental crust in the nearby source showing the close relationship to northern Gondwana prominent in West Africa and Amazonia.
The Austroalpine domain contains the type locality of eclogites, but their protolith age is unknown except for a Permian metagabbro. Therefore, we studied the non-gabbroic eclogites from the Saualpe-Koralpe Complex (SKC) representing meta-basalts within a coherent continental rift fragment subducted during the Cretaceous. Zircon laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb dating revealed protolith ages of 283 ± 5 Ma, 255 ± 3 Ma, 251 ± 3 Ma, and 241 ± 3 Ma. Magmatic zircons exhibit 176Hf/177Hf ratios of 0.282935−0.283090 and juvenile εHf(t) values of +10 to +17.4. Typical normal mid-oceanic ridge basalt (N-MORB) geochemistry is established. The SKC host metasedimentary rocks are interpreted as Variscan continental crust close to the margin of the oceanic Meliata basin and were affected by Permian metamorphism. Metamorphic zircon of one SKC eclogite yields an age of 87−93 Ma. These results combined with previous data are used to present a new model for the tectonic evolution of the distal Austroalpine unit associated with the Meliata Ocean in a Wilson cycle: The SKC represents a distal continental margin during Permian to Middle Triassic rifting, which was intruded by few gabbro bodies and numerous basaltic sills and mostly Permian pegmatites. In contrast, structurally separated thick Triassic sedimentary cover successions of the Austroalpine domain lack any magmatism, excluding SKC as Triassic basement of the sedimentary cover successions. The present eclogite-bearing piece of continental crust adjacent to the Meliata oceanic lithosphere was subducted to mantle depth during Late Cretaceous and then exhumed.
There is a controversy regarding the amalgamation of Xing'an and Songnen Blocks along the Hegenshan-Heihe Suture (HHS) in the eastern Central Asian Orogenic Belt (CAOB). To solve this problem, we performed detailed study on the granites from the Zhangdaqi area, adjacent to the north of the HHS in the northern part of the Great Xing'an Range, NE China. Geochemically, the granites in the study area are metaluminous-weak peraluminous and high-K calc-alkaline series. Trace elements of the granites show that LREEs are relatively enriched, while HREEs are relatively deficient and obvious REE fractionation. The granites are characterized by obvious negative Eu anomalies, meanwhile, they are relatively enriched in Rb, K, Th and depleted in Ba, Nb, Sr, P, Ti. All the geochemical features suggest that the granites in the Zhangdaqi area are aluminum A-type granites. The zircon LA-ICP-MS U-Pb ages of these granites are 294-298 Ma, indicating that they formed in the Early Permian. These granites also have positive ε Hf (t) values (8.4-14.2) and a relatively young two-stage model age between 449 Ma and 977 Ma, implying that the magma was derived from the re-melting of the Early Paleozoic-Neoproterozoic juvenile crust. Combined with geochemical characteristics (Nb/Ta ratios of 9.0-22.2, and Zr/Hf ratios of 52.3-152.0), we believe that the magmatic source area is a mixture of partial melting of the lower crust and depleted mantle. A-type granites and bimodal volcanic rocks along the Hegenshan-Heihe Suture formed during the Late Carboniferous-Early Permian, indicating that the HHS between Xing'an and Songnen Blocks closed in the late Early-Carboniferous. Subsequently, the Zhangdaqi area was in a post-orogenic extensional environment from Late Carboniferous to Early Permian and resulted in the formation of the A-type granites.
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