Zircon Xenocrysts Research Articles

Defining the contact zone between Archean-Proterozoic terranes with distinct geologic histories: The scenario from eastern Botswana

Understanding when Archean and Proterozoic terranes of distinct geologic histories became proximal to each other is an important topic to investigate. This study aims to define the contact zone between the southern Motloutse Complex, an Archean medium-grade terrane with prominent Neoarchean-Mesoarchean rock units, and the northern Mahalapye Complex, a Paleoproterozoic high-grade granitoid-gneiss terrane, in eastern Botswana. The contact zone between the two terranes is obscured by cover rocks. Field reconnaissance aided with remote sensing and aeromagnetic imaging delineated the NW-SE trending Radisele-Mogome corridor beneath the cover rocks. Tonalite gneiss with amphibolite and layered ultramafic rock xenoliths dominate the NE part, and are common rock types in the southern Motloutse Complex. Granodiorite with diorite gneiss xenoliths dominate the SW part, and are extensions of the NW-SE elongate c.2.04 Ga Mokgware pluton in northern Mahalapye Complex. The c.2.02 Ga Mogome granite intrudes across the contact zone, and contains xenoliths of rocks from both complexes. Shear deformation and metasomatism variably overprint the rocks along the contact zone. U-Pb zircon and 40Ar/39Ar amphibole–biotite geochronology were carried out on representative rock types to understand the chronology of events along the contact zone. The 2056 ± 40 Ma (MSWD = 5.2; n = 56) diorite gneiss from the NE margin of the contact zone contains c.2.6 Ga and c.2.9 Ga xenocrystic zircons. No Archean zircons were found in the 2041 ± 88 Ma (MSWD = 8.4; n = 13) diorite gneiss from the SW margin of the contact zone. The 2036 ± 12 Ma (40Ar/39Ar amphibole) and 2039 ± 11 Ma (40Ar/39Ar biotite) ages, respectively from the NE and SW margins, argue for the c.2.04 Ga age to represent the first common event along the contact zone. Shear deformation and metasomatism are respectively dated at 1987 ± 7 Ma and 1951 ± 11 Ma (40Ar/39Ar biotite), and account for the Pb-loss in dated zircons. It is argued that the Archean Motloutse Complex and Paleoproterozoic Mahalapye Complex terranes came together at c.2.04 Ga, with the emplacement of syntectonic Mokgware granodiorite. The results highlight the factors that are important to define the contact zone between Archean-Proterozoic terranes with distinct geologic histories.

Carboniferous magmatic records of central Mongolia and its implications for the southward subduction of the Mongol–Okhotsk Ocean

ABSTRACT The now-extinct Palaeozoic to Mesozoic Mongol–Okhotsk Ocean is evidenced by the Mongol–Okhotsk suture, which stretches from central Mongolia to the Sea of Okhotsk. The geodynamics of southward subduction of the Mongol–Okhotsk Ocean remain enigmatic, especially whether the subduction began during the Carboniferous is highly debated. In this paper we present new zircon U–Pb ages and whole-rock geochemical data for the Carboniferous magmatic rocks in Northeast Mongolia. Zircon U–Pb dating indicates that the Adaatsag dacite, Northern Bayanjargalan gabbro-diorite and Bagannur pluton were emplaced at ca. 325 Ma, ca. 316 Ma, and ca. 305 Ma, respectively. Occurrences of these rocks together constitute a Carboniferous magmatic belt to the northwest of the Middle Gobi volcanic-plutonic belt. The ages of trapped zircon xenocrysts within the Bagannur pluton indicate that the basement of this magmatic belt is the Ereendavaa terrane. Geochemical analysis indicates that these magmatic rocks were formed in an Andean-type active margin, which provides robust evidence for southward subduction of the Mongol-Okhotsk Ocean during the Late Carboniferous. By combining all the available data, we argue that the onset of the Central Mongolia–Tsagaan Uul continental collision during the Late Carboniferous – Early Permian may have triggered the beginning of an active margin of the Mongol-Okhotsk Ocean along the northern coast of the Central Mongolia microcontinent.

Direct dating of podiform Chromitite: U-Pb (Zircon, Rutile) and 40Ar/39Ar (Pargasite) evidence from Tiébaghi Cr deposit (New Caledonia)

Dominantly Mesozoic to Proterozoic xenocrystic zircons extracted from Tiébaghi chromitite coexist with a well-defined population (30%) of Eocene magmatic grains (U-Pb: 48.2 ± 1.1 Ma). Since Eocene zircons are absent in the peridotite host rock and are accompanied by rare rutiles with the same age range, it is suggested that both are derived from the circulating melt from which the chromite ore was generated. This hypothesis is strengthened by a similar 40Ar/39Ar age of 46 ± 2 Ma on amphibole that is closely associated with chromitite. These data confirm the Eocene age and supra-subduction setting of the Tiébaghi podiform chromitite, which thus was formed by interaction of peridotite and mafic melt in the final stage of the forearc magmatic event that occurred between 55 Ma and 47 Ma. Modelling the melt associated with chromite formation using zircon trace-elements and REE-zircon partitioning coefficients suggests forearc basalt as a likely candidate and the felsic by-product of peridotite-melt interaction a source for early Eocene magmatic zircons.

Archean to Paleoproterozoic crustal evolution of the southern Yangtze Block (South China): U–Pb age and Hf-isotope of zircon xenocrysts from the Paleozoic diamondiferous kimberlites

Crustal zircon xenocrysts from mantle-derived magmatic rocks have the potential to probe the deep crust. Here we present integrated U–Pb dating and Hf-isotope analyses of zircons from a Paleozoic diamondiferous kimberlite dike in the Maping area of Zhenyuan County (southeastern Guizhou Province), with implications for the tectonothermal evolution of unexposed continental crust beneath the southern Yangtze Block, South China. All zircons (n = 236) show a wide range of U–Pb ages between Mesoarchean and middle Carboniferous. Among them, 96 zircons with 90–110% concordance yield concordant ages from 2942 ± 8 Ma to 342 ± 2 Ma, and form major age peaks at ∼2.9 Ga, ∼2.6 Ga and ∼2.0 Ga. The overwhelming majority of zircons are dominated by Mesoarchean–Paleoproterozoic U–Pb ages regardless of their concordance degrees. The zircon populations mainly consist of magmatic zircons, with minor metamorphic grains (Th/U < 0.10). The youngest magmatic zircon (Th/U = 0.43) with a well concordant 206Pb/238U age of 342 ± 2 Ma (M1-16) is interpreted as the maximum emplacement age of the Maping diamondiferous kimberlites. Most zircons with pre-eruption ages are considered to be xenocrysts, and they may be derived from the deep-seated continental crust through which kimberlite host magmas have passed. Their U–Pb ages and Hf isotopic compositions suggest the possible existence of a highly evolved Archean basement beneath the southern Yangtze Block, South China, which is much older than its known surface rocks. A lot of magmatic zircon xenocrysts reveal complex Precambrian crustal evolution in the southern Yangtze Block. These processes involved the important growth of continental crust at 2.6–2.5 Ga, and in the meantime, crustal reworking could be intermittently proceeding at 3.0–2.6 Ga. In addition, a group of xenocrystic zircons are identified to be of metamorphic origin, indicating that the proposed Archean basement beneath the southern Yangtze Block likely experienced a metamorphic event around 2.0 Ga. This geologically significant episode is consistent with the well-developed coeval metamorphism in other places of the Yangtze Block (e.g., Kongling Terrane), which has been considered to link to the assembly of the Paleoproterozoic Nuna/Columbia supercontinent. Our zircon data implies that the unexposed Archean basement beneath the southern Yangtze Block was affected by multiple thermal activities.

The Role of Syn-Extensional Lamprophyre Magmatism in Crustal Dynamics—the Case of the Menderes Metamorphic Core Complex, Western Turkey

Abstract The Menderes Massif in Turkey represents one of the largest metamorphic core complexes in the world. It is regarded as a section of lower continental crust exhumed along low-angle detachment faults in the Late Miocene during a period of extension that affected the entire Aegean province. Syn-extensional magmatic activity within the Menderes metamorphic core complex is predominantly felsic forming several plutons, whereas mantle-derived magmatism has not been known so far. Here, we present a detailed study of the petrology and geochemistry of previously unreported mafic to intermediate lamprophyres within the Menderes Massif and assess their role in the geodynamic evolution of the core complex. The Menderes lamprophyres are mostly kersantites, with 49–60 wt % SiO2, 3.2–8.4 wt % MgO, 100–360 ppm Cr, 32–132 ppm Ni and Mg# of 37–50. Positive Pb and negative Ti–Nb–Ta anomalies suggest a clear orogenic affinity. Isotopes of Sr and Pb are relatively radiogenic (87Sr/86Sr = 0.70609–0.71076; 206Pb/204Pb = 18.88–19.03, 207Pb/204Pb &amp;gt; 15.71), while Nd is unradiogenic (εNd = −1.4 to −3.2). Most phenocrysts are sharply zoned with a primitive core (Mg# 77–85, up to 0.95 wt % Cr2O3 in clinopyroxene; Mg# 72–76 in amphibole) and a more evolved rim (Mg# 68–74, &amp;lt;0.25 wt % Cr2O3 in clinopyroxene; Mg# 69–71 in amphibole). Trace element ratios between different cores may vary significantly (e.g. Dy/Yb 2–5 in amphiboles), whereas rims show less variation but are more enriched than the cores. U–Pb dating of zircons provides an age of 15 Ma for the lamprophyres, coeval with the syn-extensional granite magmatism. The Hf isotopic composition of magmatic zircons is variably unradiogenic (176Hf/177Hf15Ma = 0.28248–0.28253, εHf15Ma = −8.6 to −10.5), while zircon xenocrysts with dominantly Cadomian and older ages show highly variable Hf isotopic signatures at the time of lamprophyre emplacement (εHf15Ma = −7.6 to −46.7). The orogenic geochemical signature of the lamprophyres’ parental melts is similar to nearby orogenic lavas from the West Anatolian Volcanic Province. Variation in bulk-rock εNd and in Dy/Yb ratios of phenocryst cores reflect moderate mantle heterogeneity. The chemical heterogeneity of phenocrysts and zircon εHf values implies intense hybridisation of proto-lamprophyre melts with felsic crustal melts, most probably derived from the melting of augen gneisses of the Menderes basement. We propose that fluid released from the lamprophyre primary melt had a decisive impact on crustal melting and the formation of granitic plutons within the Menderes core complex.

Late Mesozoic volcanism in the southern Xiong'er Mountains, eastern Qinling: Partial melting of the subducted Qinling crust underneath North China

Abstract Volcanism along the continental margins is a powerful tool with which to probe the orogenic processes and crustal components underlying orogenic belts. In this study, we report the zircon ages and geochemical compositions of volcanic rocks exposed in the Luanchuan area, along the southern margin of the North China block. In previous studies, these volcanic rocks were reported to be products of Paleoproterozoic (ca. 1.75 Ga) volcanism related to rifting in the Xiong'er Mountains. Our zircon age data reveal that most volcanic rocks contain late Mesozoic zircon grains of magmatic origin, suggesting that Late Jurassic to Early Cretaceous (ca. 150–120 Ma) volcanism must have occurred in the southern Xiong'er Mountains, coeval with emplacement of large granitoid plutons along the southern margin of the North China block. These late Mesozoic volcanic rocks mainly consist of trachyandesite, andesite, dacite, and rhyolite. They vary in their major-element compositions but are relatively uniform in their normalized patterns of trace and rare earth elements and have consistently enriched Nd isotopic compositions. Numerous inherited and/or xenocrystic zircon grains of mostly Paleoproterozoic and occasionally Neoproterozoic to early Mesozoic ages were found in all of the volcanic rocks, indicating complex magma sources and/or diverse crustal rocks overlying the magma chambers. The Paleoproterozoic zircon grains originated from the Paleoproterozoic volcanic and basement rocks in the southern North China block. Notably, some volcanic rocks contained early Mesozoic, early Paleozoic, and Neoproterozoic zircon grains of both metamorphic and magmatic origin that indicate derivation from the North and South Qinling blocks, which recorded thermo-tectonic events during the closure of the Shangdan and Mianlue oceans in the early Paleozoic and early Mesozoic. Therefore, we propose that these late Mesozoic volcanic rocks originated from partial melting of crustal rocks and are composed partly of the subducted basement of the Qinling orogenic belt underneath the southern Xiong'er Mountains due to asthenospheric upwelling in an extensional setting.

Tibetan Plateau growth linked to crustal thermal transitions since the Miocene

Abstract The topographic transition of central–northern Tibet since the early Miocene has created a consistently high and flat plateau similar to that of today. However, to date, the associated deep crust and/or /mantle events are poorly understood, mainly due to an early Miocene metamorphic–magmatic lull within the Qiangtang Block. To address this issue, we undertook a study of crustal xenoliths and zircon xenocrysts in 6.0–2.3 Ma lavas in the Qiangtang Block. The occurrence of 22.6–12.9 Ma high-temperature–low-pressure granulite xenoliths implies that the middle crust of the block has been very hot since that time. Zircon xenocrysts and granitic xenoliths from 6.0–2.3 Ma lavas were studied and shown to have high δ18O values, which supports Miocene crustal melting and the formation of unexposed, coeval felsic plutons. Combined with paleoelevation data from the Tibetan Plateau, our results suggest that the early Miocene cold–hot thermal transition of the middle–lower crust was near-synchronous with topographic evolution from high-relief mountains to a flat plateau, which supports crustal flow as the main topographic smoothing mechanism for central–northern Tibet.

Zircon xenocrysts in Late Cretaceous magmatic rocks in the Kermanshah Ophiolite: link to Iran continental crust supports the subduction initiation model

ABSTRACT Geochronological and geochemical data show that Late Cretaceous Zagros ophiolites were generated by forearc seafloor spreading during a subduction initiation event along the southwestern margin of Iran. These ophiolites define a ~ 3000 km long belt passing from Cyprus to Turkey, Syria, Iran, the UAE, and Oman. Ten samples of plagiogranites, amphibole gabbros and diorites of the Kermanshah ophiolite yield U-Pb zircon magmatic ages indicating ophiolite formation at 98 to 96 Ma. Some Kermanshah rocks also contain abundant zircon xenocrysts with age peaks (in order of decreasing size): Triassic (~247 Ma), mid-Paleoproterozoic (~2150 Ma), Late Paleoproterozoic (~1850 Ma), Late Ediacaran (~560 Ma), Early Cretaceous (~140 Ma), and Tonian (~850 Ma). Similar age peaks are seen in magmatic and detrital rocks of Iranian crust to the north. The xenocrysts have εHf(t) values from +6.3 to – 21.6, corresponding to depleted mantle Hf model ages (TDM) of 0.6 to 3.6 Ga. The xenocryst age peaks are similar to those of zircons in Iranian continental crust. The occurrence of zircon xenocrysts in the Kermanshah ophiolite suggests that this formed adjacent to the Iranian continent, as expected for Late Cretaceous subduction initiation.

Zircon Xenocrysts from the Shaka Ridge Record Ancient Continental Crust: New U-Pb Geochronological and Oxygen Isotopic Data

Over the past two decades, a significant number of discoveries of ancient zircon xenocrysts in ocean-floor magmatic rocks have been reported. These findings provide compelling evidence for the presence of ancient continental crust within young oceanic lithosphere. Almost all finds of ancient zircon xenocrysts within oceanic crust are from the Mid-Atlantic Ridge. For other localities, however, similar data are very limited. This report presents the first age determinations (U-Pb, SHRIMP-II) and isotope-geochemical data (oxygen, trace and rare earth elements) for zircon xenocrysts from gabbro-diorites of the Shaka Ridge, in the vicinity of the Shaka fracture zone, near the western end of the Southwest Indian Ridge. The work is based on a study of bottom rock material dredged during expeditionary research on the R/V “Akademik Fedorov” (Russia) in 2016. The U-Pb isotope system of the zircon xenocrysts recorded a crystallization age of ∼2.8 Ga at an upper discordia intercept and an age of ∼600 Ma interpreted as the timing of a superimposed thermal event at a lower discordia intercept. The zircon xenocrysts show geochemical signatures of magmatic origin, i.e., fractionated REE distribution spectra with an increase in chondrite-normalized values from light to heavy REE, positive Ce anomalies and negative Eu anomalies, and high Th/U ratios (0.59–7.77). In discrimination diagrams based on a series of inter-element relationships, zircon compositions fall into the fields of zircons from rocks of continental crust, mostly granitoids. The Li content of the zircons is high (1.8 ppm–50 ppm), adding further evidence to their derivation from rocks of continental crust. During their residence within young oceanic crust, the zircon xenocrysts experienced alterations under the influence of submarine high-temperature hydrothermal fluids, which selectively affected the distribution of trace elements in the zircons and reduced the δ18O values to 1.75‰–3.15‰. The presence of obviously older zircons in Shaka gabbro-diorites clearly demonstrates the presence of ancient continental fragments and their recycling into the mantle at the western end of the Southwest Indian Ridge.

Late carboniferous continental arc magmatism in the southeastern Central Asian Orogenic Belt: insights from the Erenhot granitic pluton, Inner Mongolia

The closure time of the Hegenshan Ocean and its suturing scenario is debated, which hinders our understanding of the architecture and tectonic evolution of the southeastern Central Asian Orogenic Belt. We present an integrated study of in-situ zircon U-Pb isotopic ages and Hf isotope compositions, whole-rock geochemistry and Sr-Nd isotopes on the less-studied Erenhot granitic pluton in central Inner Mongolia. The <i>ca.</i> 303 to 300 Ma Erenhot pluton is composed of monzogranites, which are high-K calc-alkaline rocks and show I-type affinities. Moreover, positive correlation between Ba and Sr, along with the notably positive Rb and Pb anomalies, negative δEu and depletion in Ba, Eu, Nb and Ti, imply crystal fractionation played an important role in the magma evolution. The pluton also shows relative enrichment in light rare earth elements and large ion lithophile elements and depletion in high field strength elements, typical features of subduction-related magma. Isotopically, predominantly positive whole-rock ε_Nd(t) (0.9–3.8) and zircon ε_Hf(t) (4.50–13.66) values indicate a dominant juvenile crustal source with minor older crustal contamination. Given the mostly Proterozoic model ages (1318–543 Ma) and the presence of xenocrystic zircons, we infer that the Erenhot pluton probably was formed in a continental arc setting. Combined with the available published data, it is proposed that an east-west trending continental arc developed and was accreted onto the Uliastai Continental Margin in the Carboniferous, resulting from the northward subduction of the Hegenshan Ocean. The late Carboniferous continental arc-related magmatism (<i>ca.</i> 303–300 Ma) at Erenhot probably witnessed the waning stage of Hegenshan oceanic lithosphere subduction beneath the Uliastai Continental Margin.

Early carboniferous rifting of the Harlik arc in the Eastern Tianshan (NW China): Response to rollback in the southern Altaids?

Field, geochronological, geochemical and Sr-Nd isotopic analyses are applied to late Paleozoic gabbro-diorites and monzogranites in the Hulugou-Miaoergou regions, Harlik arc, in order to provide constraints on the tectonic evolution of the Eastern Tianshan orogen in the late Paleozoic. LA-ICP-MS zircon U-Pb ages show that the first pulse of gabbroic magmatism occurred at 348 ± 4 Ma, accompanied by simultaneous dioritic (342 ± 3 Ma) and monzogranitic (343 ± 5 Ma) magmatism, and the second pulse of gabbroic magmatism was at 334 ± 3 Ma. Most of the gabbros are medium- to high- K calc-alkaline in composition, and show enrichments in light rare earth elements (LREE) and large ion lithophile elements (LILE), but depletions in high field strength elements (HFSE, especially Nb and Ta). In combination with their juvenile isotopic signature (initial ⁸⁷Sr/⁸⁶Sr = 0.70345–0.70380, εNd(t) = 4.5–6), the geochemical features suggest that both pulses of gabbroic magmatism were likely derived from partial melting of asthenosphere facilitated by a flux from a subducting slab. The diorites also display the arc-related geochemical characteristics and juvenile isotopic signature (initial ⁸⁷Sr/⁸⁶Sr = 0.70355–0.70358, εNd(t) = 4.3–4.7), coupled with their intimate relationship with the gabbros indicate that they formed by fractional crystallization of clinopyroxene, plagioclase and amphibole from the first pulse of gabbroic magmatism. The monzogranites have relatively high A/CNK values (1.04–1.1) and are weakly to moderately peraluminous. Considering the presence of zircon xenocrysts dated at 510 to 450 Ma in the monzogranites, and their moderate molar Al₂O₃/(MgO+FeO^T) and molar CaO/(MgO+FeO^T) ratios, partial melting of supracrustal rocks of probable late Ordovician age was most likely the cause of their genesis and heterogeneity. These three different magmas (gabbroic, dioritic and monzogranitic) were probably extracted from a deep crustal hot zone. The primitive basaltic magmas continuously intruded the deep crust where they solidified, fractionated, assimilated, and heated the crust, generating in turn the peraluminous and A-type granitoids, similar to the magmas in the Lachlan orogen that intruded during progressive slab rollback. Based on all available evidence, we propose that southward slab rollback of the subducting Paleo-Asian Ocean in the early Carboniferous was responsible for the progressive emplacement of these different magmas, which eventually resulted in rifting of the Harlik arc.

Giant Quartz Veins of the Bundelkhand Craton, Indian Shield: New Geological Data and U-Th-Pb Age

Giant quartz veins are widespread on the Bundelkhand Craton of the Indian Shield which precise ages with a SHRIMP-II, U-Th-Pb isotope are quantified in this article. Their relative geological age is well-documented: they cut the Paleoproterozoic (2150–1800 Ma) sediments of the Bijawar Group and are overlain by Proterozoic (1670–1030 Ma) sediments at the base of the Upper Vindhyan Supergroup. U-Th-Pb dating of zircon grains from a quartz vein was carried out to assess major event of their formation as 1866 ± 12 Ma. This data is consistent with the relative geological age of the veins. In addition, the quartz veins were shown to contain 2.86, 2.7, and 2.54 Ga xenocrystic zircon grains. Rocks with these ages are abundant in the craton. The formation of a giant quartz vein swarm is associated with the deformation of the Bundelkhand Craton lithosphere during 1.9–1.8 Ga ago triggered by compression caused by collision processes at the western flank of the Columbia Supercontinent on one side and plume activity on the other.

Evolution of the Neoarchean lower crust beneath Western Liaoning of the North China Craton: Evidence from U-Pb-Hf-O isotopes in zircon

A comprehensive analysis of U-Pb geochronology and Hf-O isotopes of zircons from granulite xenoliths hosted in the Cretaceous basalt and zircon xenocrysts in the Jurassic andesitic volcanic rocks of Western Liaoning within the North China Craton (NCC) provides an opportunity to understand the evolution of the lower crust in the region. Zircons from these granulites yield 207Pb/206Pb ages of 1678–2646 Ma and consistent upper intercept ages of 2514–2544 Ma. In contrast, zircon xenocrysts yield two periods of 207Pb/206Pb ages of Neoarchean (2500–2591 Ma) and Paleoproterozoic (1733–2499 Ma), which produce consistent upper intercept ages of 2543–2549 Ma and lower intercept ages of 1761–1789 Ma, respectively. The Neoarchean zircons from the granulites and xenocrysts generally have similar positive εHf(t) values resulting in Hf depleted mantle model ages (TDM) of 2.5–2.9 Ga with a mode of 2.7 Ga and variable δ18O (5.05‰ to 8.81‰) values. However, the δ18O values of zircons from the xenocrysts are a little bit higher than those of zircons from granulites (peak of 8.00‰ for the xenocrysts and 6.00‰ for the xenoliths). Some Paleoproterozoic zircon xenocrysts have negative εHf(t) values (−12.8 to −9.39) and similar 176Hf/177Hf ratios and δ18O values to those of the Neoarchean zircons, whereas the other Paleoproterozoic zircons show chondritic εHf(t) (−4.34 to 1.16) and higher δ18O (7.70‰ to 10.19‰) values. All these data demonstrate that the primordial lower crust beneath Western Liaoning originally formed in the Neoarchean, perhaps around 2.6–2.7 Ga. The ubiquitous presence of ca. 2.5 Ga magmatic zircons illustrates that an important melting event indeed happened in the lower crust beneath Western Liaoning, similar to the thermal event throughout the entire NCC. In addition, the Neoarchean lower crust underwent another important thermal event during the Paleoproterozoic, which was considered to be associated with the final amalgamation of the Western and Eastern blocks of the NCC. Thus, we can conclude that there indeed existed an Archean lower crust beneath the Western Liaoning until to the Late Mesozoic, and this Archean lower crust underwent a similar multiple-stage (Neoarchean and Paleoproterozoic) evolutionary history as those throughout the NCC.

Geochronology and geochemistry of the Huntington Formation, Olds Ferry terrane, Blue Mountains province, northern U.S. Cordillera: Implications for accreted terrane correlation and assembly

AbstractThe Olds Ferry terrane is the more inboard of two accreted volcanic arc terranes in the late Paleozoic–early Mesozoic Blue Mountains province of the northern U.S. Cordillera. We present geologic, geochronologic, and geochemical data from the volcano-sedimentary Huntington Formation of the Olds Ferry arc that place the terrane within a firm temporal and tectonomagmatic context, and establish its identity as a fringing arc terrane along the Triassic to Early Jurassic Cordilleran margin. The Huntington Formation is divided into two unconformity-bounded informal members: a Norian (ca. 220 Ma) lower member comprising a sequence of mafic-intermediate volcanics, massive volcaniclastic breccias, and minor carbonates deposited unconformably onto the 237.7 Ma Brownlee pluton and intruded by the 210.0 Ma Iron Mountain pluton; and a Rhaetian through Pleinsbachian (&amp;lt;210–187.0 Ma) upper member composed of massive conglomerates, abundant rhyodacite to rhyolite effusive and pyroclastic flows, and interlayered sandstone turbidites, deposited with angular unconformity onto the lower member. An erosional hiatus and regional tilting produced an angular unconformity separating the Huntington Formation from the overlying basal conglomerates of the late Early to Middle Jurassic Weatherby Formation of the Izee forearc basin transgressive onlap sequence. Huntington Formation volcanic rocks are isotopically enriched relative to depleted mantle and coeval igneous rocks in the outboard Wallowa terrane. A temporal evolution to more radiogenic 87Sr/86Sr ratios (0.7036–0.7057) and εNd values (+5.4 to +3.1) in the upper member volcanics suggests increasing involvement of continental-derived material in their petrogenesis. Precambrian xenocrystic zircons in both lower and upper member volcaniclastic rocks strongly support a proximal location of the Olds Ferry terrane to cratonal North America during much of its history. The chronology and tectonostratigraphic architecture of the Olds Ferry terrane allows its robust correlation to other fringing-arc terranes along the U.S. and Canadian Cordillera.

U-Pb zircon xenocrysts dating as a proxy to assess volcanic assimilation and the underlying crust, Cretaceous Jaguarão Formation, RS, Brazil

The Early Cretaceous volcanic Jaguarão Formation formed duringthe early stages of Gondwana breakup. These felsic rocks characteristically enclose numerous crustal xenoliths. Fieldwork, petrographic studies, and laser ablation multicollector ion coupled plasma mass spectrometer (LA-ICP-MS) U-Pb analyses were carried out to evaluate the xenoliths of the Jaguarão Formation. A base-to-top distribution of xenoliths in the different flows of the volcanic pile indicates that earlier flows incorporated centimeter-sized fragments of local underlying gneiss, granite, and quartz veins, with limited assimilation. In the middle part of the volcanic pile, the size of the xenoliths is smaller and assimilation textures are more frequent. At this same level, reequilibrated orthopyroxene and plagioclase antecrysts were recognized, suggesting fractionation coeval with assimilation in a lower magmatic chamber prior to extrusion. At the top, no visible xenoliths occur. U-Pb ages of the zircon xenocrysts served as a proxy for assessing the crust underlying the volcanic rocks. Most of the ages obtained are similar to the ages of the Eastern Dom Feliciano Belt cropping out nearby, ranging from 680 to 620 Ma, with a subordinate group ranging from 580 to 570 Ma, suggesting a shallow crust contribution. The presence of few Paleoproterozoic and Mesoproterozoic (1.0-1.2 Ma) zircons, however, suggeststhat the underlying terrain may be more complex..

Mesoarchean trondhjemitic continental nucleus and pre-plate tectonic crustal-mantle interactions of the Western Shandong Complex, North China Craton

Plate tectonics play an important role in crustal-mantle interactions since late Archean, yet detailed processes of pre-plate tectonic crustal-mantle interactions remain enigmatic. In the Western Shandong Complex of the North China Craton, magmatic rocks recorded successive tectonic transition processes from vertical (plume) to lateral (subduction) during the early Neoarchean. These magmatic records, together with our newly discovered a pre-Neoarchean continental nucleus in the Western Shandong Complex, documented key information on pre-plate tectonic crustal-mantle interactions. In this study, petrology, whole-rock geochemical and Sm–Nd isotopic data, and zircon U–Pb and Lu–Hf isotopes are reported for Neoarchean supracrustal and intrusive rocks for the Western Shandong Complex. Early Neoarchean komatiitic basalts and ultramafic rocks (∼2.77 Ga) were plume-related and generated by crustal contamination of fractional crystallized komatiitic magma. A series of Mesoarchean (∼3.0–2.8 Ga) xenocrystic zircon grains were discovered in the Late Neoarchean charnockites (∼2.56–2.52 Ga), indicating contributions of continental crustal material earlier than regional oldest magmatic records. Based on our new geochronological and geochemical data, a trondhjemitic Mesoarchean (∼3.1–2.8 Ga) continental nucleus was identified and reported for the first time in the Western Shandong Complex. During the early stage of the early Neoarchean, this continental nucleus dismembered during the bottom-up plume processes and the correlated plume-continental nucleus interactions occurred and ubiquitously documented in regional Neoarchean magmatic rocks with different degrees. The plume-continental nucleus interactions consequently resulted in the opening of an oceanic basin and different continental evolutionary processes during the subsequent lateral subduction in the early Neoarchean. The buoyant oceanic plateau was possibly dragged to the trench and blocked subsequent subduction and led to a ∼40 Myr magmatic hiatus between 2.60 Ga and 2.56 Ga.

Zircon U–Pb ages in El Chaltén Jurassic rhyolites (Chon Aike Silicic LIP, Argentina) reveal xenocrystic remnants of an unexposed lower basement of the Paleo-Pacific Gondwana margin

We investigate the Jurassic volcanic rocks of the El Quemado Complex in the El Chaltén region (Argentina), which correlate with the youngest eruptive event of the Chon Aike Silicic LIP in southern Patagonia. New zircon crystallization ages of the various silicic units were obtained using U–Pb geochronology measured by in-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The average ages for 12 samples range from 147 to 152 Ma. Xenocrystic zircon cores of Jurassic igneous zircons are characterized by complex cathodoluminescent textures wherein growth surfaces are truncated by dissolution surfaces that are in turn overgrown by euhedral growth zones that exhibit oscillatory magmatic zoning. The cores have a wide range of ages, from 185 Ma to 1.3 Ga. We combine our analyzed xenocryst zircon ages preserved in the El Quemado Complex rhyolites with a compilation of published xenocryst zircon ages documented from Cenozoic to Paleozoic volcanic and plutonic rocks throughout southern Patagonia. The combined datasets suggests three prominent geologic features not currently exposed on the present day surface: (1) widespread abundant Triassic through Permian (200–300 Ma) xenocrystic ages throughout the present day Andean region, which connect to current exposures in Northern Patagonian and that of the Antarctic Peninsula, marking the Paleo-Pacific Gondwanan margin, (2) the existence of widespread Early-Middle Ordovician to Cambrian ages throughout southern Patagonia, and (3) the presence of an old (∼1.0 Ga) crust below that of the Deseado Massif and Tierra del Fuego, which likely extends beneath the present-day Andes.

Zircon xenocryst Hf-O isotopic compositions in the Qiyugou Au orefield: A record of Paleoproterozoic oceanic slab subduction in the Trans-North China Orogen

The Trans-North China Orogen (TNCO) is suggested to form under a transition from subduction to collision from 2500 Ma to ∼ 1850 Ma, and the Taihua TTG gneiss in the Xiong’ershan district is thought to have formed in an Andean-type continental arc setting or island arc setting during this geological event. However, to date, the geochemical record of the subducted oceanic slab is still lacking. This study focuses on Hf-O isotopic analyses of zircon xenocrysts from the Qi189 hornblende monzogranite, Qiyugou Au orefield, TNCO. The results reveal that zircon xenocrysts with ages from 2304 Ma to 1934 Ma generally have low-δ18O values from 2.66‰ to 3.83‰ and high-εHf(t) values from 2.7 to 7.8. These zircon grains were generated from remelting of high-temperature altered gabbroic rocks of the subducted oceanic slab during the transition from compression to extension. In this paper, we present new evidence for Paleoproterozoic oceanic slab subduction in the TNCO.

First Evidence of the Post-Variscan Magmatic Pulse on the Western Edge of East European Craton: U-Pb Geochronology and Geochemistry of the Dolerite in the Lublin Podlasie Basin, Eastern Poland

The U–Pb measurements of youngest, coherent group of zircons from the Mielnik IG1 dolerite at the Teisseyre-Tornquist margin (TTZ) of East European Craton (EEC) in Poland yielded age of 300 ± 4 Ma. Zircon dated an evolved portion of magma at the late stage crystallization. It is shown that this isolated dyke from the northern margin of the Lublin Podlasie basin (Podlasie Depression) and regional dyke swarms of close ages from the Swedish Scania, Bornholm and Rügen islands, Oslo rift, Norway, and the Great Whine Sill in northeastern England, were coeval. They have been controlled by the same prominent tectonic event. The Mielnik IG1 dolerite is mafic rock with Mg-number between 52 and 50 composed of the clinopyroxene, olivine-pseudomorph, plagioclase, titanite, magnetite mineral assemblage, indicating relatively evolved melt. This hypabyssal rock has been affected by postmagmatic alteration. The subalkaline basalt composition, enrichment in incompatible trace elements, progressive crustal contamination, including abundance of zircon xenocrysts determines individual characteristics of the Mielnik IG1 dolerite. The revised age of dolerite, emplaced in vicinity of TTZ provides more evidences documenting the reach of the Permo-Carboniferous extension and rifting accompanied by magmatic pulses, that were widespread across Europe including the margin of the EEC incorporated that time into the broad foreland of the Variscan orogen.

Solid-phase transfer into the forearc mantle wedge: Rutile and zircon xenocrysts fingerprint subducting sources

Voluminous flux of a hydrous component from subducting oceanic lithosphere into the forearc mantle and its contribution to arc magmas have long been recognised but the transfer of solid-phase materials has received less consideration. Although xenocrystic zircons are known from some arc magmas and ophiolitic chromitites their origins remain enigmatic. How and when such materials are transferred into the overlying wedge of mantle lithosphere and the length of their residency therein are unclear. Using zircons and rutiles recovered from ophiolitic rocks in New Caledonia, we demonstrate unambiguous association with subducting sediments and hence evidence for inter-plate transfer. The ‘fingerprints’ of age spectra for both minerals can be matched to lithospheric slab components subducted during Eocene time. Their occurrence in forearc harzburgite suggests that they were incorporated into their mantle wedge host at subduction depths beyond the onset of serpentinisation at ca. 300°C but before the ca. 600°C destabilisation temperature of the rutile U-Pb system. Relocation from slab to forearc mantle is therefore likely to have occurred at ca. 50-60 km depth within the subduction channel. Fluid-assisted conveyance of high field strength elements within solid-state accessory mineral phases into supra-subduction mantle wedges may represent a little-recognised but significant phenomenon that is both predictable and pervasive at convergent margins globally.