Plutonic rocks from the Mineiro Belt, Brazil record a delayed onset of the transition from TTG to sanukitoid-type magmatism (high Ba-Sr), starting during the Siderian magmatic lull when little juvenile magma was added to the continental crust. Rocks mostly belong to the calc-alkaline series, meta- to peraluminous and originally "I-type", meaning that oxidized magmas were formed by partial melting of subducted material. The temporal distribution and apparent secular changes of the magmas are consistent with the onset of subduction-driven plate tectonics due to an increase of the subduction angle and opening of the mantle wedge. New isotopic analyses (Sm-Nd whole rock and Lu-Hf in zircon) corroborate the restricted juvenile nature of the Mineiro Belt and confirm the genetic link between the Lagoa Dourada Suite, a rare ca. 2350 Ma high-Al tonalite-trondhjemite magmatic event, and the sanukitoid-type ca. 2130 Ma Alto Maranhão Suite. U-Pb dating of zircon and titanite constrain the crystallisation history of plutonic bodies; coupled with major and trace element analyses of the host rocks, they distinguish evolutionary trends in the Mineiro Belt. Several plutons in the region have ages close to 2130 Ma but are distinguished by the lower concentration of compatible elements in the juvenile high Ba-Sr suite.
Small volume U-Th-Pb isotope analysis of accessory minerals is a major aim of modern geochronology. Analytical techniques are constantly developed to try and date ever smaller features, such as metamorphic rims and inclusions, and more recently produce age maps. Improvements in laser ablation ICP-MS technology and methodology offer great potential in this regard, but only a few laboratories are routinely operating with laser beam diameters ≤10 µm, mostly using mutli-collector ICP-MS instruments. Here we present analytical developments using a high-sensitivity quadrupole ICP-MS instrument (Analytik Jena PQ Elite), coupled with a 193 nm ArF excimer laser (ASI RESOlution-SE), which allow for rapid small volume geochronology of minerals including zircon, monazite and baddeleyite. The smallest achievable ablation volume for precise and accurate laser ablation U-Th-Pb geochronology is largely controlled by ablation efficiency, the degree of laser induced elemental fractionation (LIEF), signal stability and the sensitivity of the mass spectrometer. We present the results of a range of tests designed to optimize ablation parameters at beam diameters ranging from 9 to 15 microns, and ablation pit depths 150 cps/ppm of 238U. U-Th-Pb isotope ratio data collected using these ablation conditions for zircon reference materials GJ-1, Temora2, and 91500 (normalized to Plesovice) are accurate with 2σ errors of 2-3% and 4-6% on 206Pb/238U and 207Pb/235U ratios respectively. These developments are opening up exciting new applications, including the dating of mineral inclusions, multiple age domains in single crystals, and thin metamorphic rims on zircon and monazite.
An improved method of U–Pb dating of vein calcite formed during deformation is used to determine the age and cause of folding along the south coast of England. Fractures arising from folding of Late Cretaceous Chalk of southern England occurred 34.7 ± 1.7 myr ago. Underlying Jurassic strata have veins within fractures with ages of 55, 48–42, 39–37, 34–31 and 25 Ma, with 34–31 Ma being the tectonic culmination. Folding was slightly younger than the age of the youngest strata in the overlying Solent Group, suggesting that folding terminated basin sedimentation. This age of north–south shortening is inconsistent with attribution to intraplate forces from the mainly younger Alps, but is plausibly a result of the Pyrenean Orogen, which evolved from 50 to 28 myr ago with a late Eocene culmination. A modified method for carbonate laser ablation inductively coupled plasma mass spectrometry U–Pb dating is presented that uses measured 232 Th and 208 Pb as a better monitor of common Pb, and it has distinct advantages over existing methods. Initial common lead determined on samples conforms closely to model compositions calculated by earlier workers, with free regressions giving the most robust dates. The modified method is applicable to structural geology, carbonate diagenesis and to dating of carbonate relevant to palaeo-environmental and archaeological studies. Supplementary material : Details of sample locations and characteristics, photographs of sample localities, microscopic images of veins, details of the experimental procedure and U–Pb data for all samples and reference materials are available at https://doi.org/10.6084/m9.figshare.c.3997086
Abstract Oyu Tolgoi is a world-class, Late Devonian porphyry district in southern Mongolia. Because of its age and geodynamic setting, it has undergone a complex geological history that includes major postmineralization magmatic-hydrothermal events in close proximity to the porphyry deposits. The propylitic alteration halos that surround the Cu-Au deposits contain widespread hydrothermal titanite, as do the younger altered volcanic and intrusive rocks. Here, we present a comprehensive laser ablation-inductively coupled plasma-mass spectrometry U-Pb study on in situ, propylitic titanite from the Oyu Tolgoi district. The results identify district-wide hydrothermal alteration episodes that coincide with known magmatic events: Devonian porphyry mineralization (~372 Ma); the intrusion of granodiorite plutons and andesite dikes in the Carboniferous (~320–310 Ma); and the emplacement of the Permian Khanbogd Granite alkaline batholith (~290 Ma). Both Carboniferous and Permian alteration events variably overprint the earlier porphyry alteration halo. Overdispersion in the U-Pb data from some samples, due to Pb and/or U mobility, makes interpretation of some titanite ages more difficult, but further exemplifies the complex alteration history of the district. We conclude that U-Pb dating of propylitic titanite is a viable means by which explorers can identify alteration within a prospect that is synchronous with potentially fertile intrusions of known age. The extent of the coeval propylitic alteration and its mineral chemistry can then be used to assess the likelihood of a major porphyry center being present.
The formation and reworking of the continental crust is of great importance for understanding the early evolution of the Earth. Combining U−Pb/Hf isotopes in zircons with whole rock geochemical and Sr-Nd isotopic data can provide new insights on petrogenetic mechanisms, timing of magmatic events, crust-mantle interactions and magma sources for crustal material. Here we present a combined dataset of in situ zircon U−Pb and Hf as well as whole-rock Sr and Nd analyses for Archean TTGs and geochemically variable high-K granitoids from the Bundelkhand Craton, India. U−Pb zircon ages reveal that the TTGs were emplaced at 3.42 Ga, 3.33 Ga and 2.72 Ga, while the high-K granites, including sanukitoids, were emplaced between 2.57 Ga and 2.54 Ga. The high-K granitoids have higher initial 87Sr/86Sr isotope ratios than the TTGs. They also display a lower range in initial eNd and eHf values (from -8 to -1 and -8.9 to +0.4, respectively) compared with the TTGs (from -4 to -1.2 and -1.6 to +4.4, respectively). The Hf depleted mantle model ages calculated for high-K granitoids are 3.19−2.86 Ga and for TTGs 3.71−3.48 Ga. The U−Pb ages and chondritic to superchondritic eHf values of the TTGs provide evidence for a long-term episodic growth of juvenile crust from depleted mantle reservoirs between 3.4 and 2.7 Ga. The strongly negative eNd and eHf values of the high-K granitoids, together with geochemical features (variable compatible and incompatible elements) indicate that they were a result of multi-stage reworking of the Paleo- to Neoarchean crust and mixing with magmas extracted from an enriched mantle source during a relatively short-lived tectonic event at the end of the Archean.
Abstract Amphibole- and clinopyroxene-bearing monzodiorites were emplaced at 340 Ma (CA-ID-TIMS zircon age), suggesting the formation of hydrous and dry magmas closely related in space and time in the NE Bohemian Massif. Hafnium and oxygen isotopes of zircon in less evolved rocks (<55 wt% SiO2) are similar between Amp and Cpx monzodiorites (εHf = −3.3 ± 0.5 and − 3.5 ± 0.8; δ18O = 6.4 ± 1.0 and 6.8 ± 0.7, respectively), consistent with a common source—a contaminated mafic magma derived from an enriched mantle. At the same time, the conditions of crystallization are distinct and zircon appears to be an excellent tool for distinguishing between hydrous and anhydrous crystallization conditions, a process that may be more ambiguously recorded by whole rock and major mineral chemistry. In particular, elements fractionated by either amphibole or plagioclase crystallization, such as Hf, Dy, and Eu, differ in zircon from amphibole- and clinopyroxene-bearing rocks, and Zr/Hf, Yb/Dy, and Eu/Dy are therefore useful indices of crystallization conditions. We show that the composition of zircon from hydrous dioritic magmas is not comparable with that of typical zircon from dioritic-granitic suites worldwide, suggesting a specific process involved in their formation. Here, we propose that fluid-present remelting of a mafic underplate is necessary to explain the rock textures as well as the composition of the whole rock, zircon, and other minerals of amphibole-bearing monzodiorites and that a similar process may control the formation of amphibole-rich dioritic rocks worldwide, including appinitic suites. Overall, we show that dioritic rocks represent snapshots of differentiation processes that occur in the early stages of magma evolution before the magma is homogenized into large-scale batholiths.
In situ U–Pb dating of monazite from granulite-facies anatectic aluminous gneisses of the hinterland of the Grenville Province (Manicouagan area) is used to constrain the age of metamorphic events. Matrix grains in these rocks show complex internal textures consistent with extensive corrosion and overgrowths which are attributed to partial dissolution of earlier monazite in anatectic melt followed by new growth during melt crystallization or subsequent fluid infiltration. The new monazite data show the following: (i) inherited “pre-Grevillian” ages up to ca. 1400 Ma in some rocks; (ii) “main Grenvillian” ages in the general range of ca. 1070–1020 Ma, with a variable spread in individual samples and a general cluster at 1070–1050 Ma; and (iii) “late Grenvillian” ages at ca. 1010–990 Ma, mostly restricted to backscatter electron (BSE)-bright rims of matrix grains. The wide age range of the main Grenvillian metamorphism suggests episodic growth of monazite over a wide time span, consistent with protracted residence of the host rocks under high-temperature conditions. The clusters in the age distribution likely represent major episodes of melt crystallization in the respective rocks, following the granulite-facies metamorphism. In contrast, the growth of the late Grenvillian monazite at ca. 1000 Ma is attributed to late fluid infiltration of the host rocks under greenschist-facies conditions, coeval with ultrapotassic magmatism. It is the first report of a late Grenvillian metamorphic overprint on granulite-facies mineral assemblages in the hinterland and is consistent with the model of extensional collapse of the orogen.
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