The Garies Wollastonite Deposit, Namaqualand, South Africa: High-Temperature Metamorphism of a Low-δ18O Skarn?

ABSTRACT The Garies wollastonite deposit is located in the Bushmanland terrane of the Namaqualand Metamorphic Province and is part of a discontinuous calc-silicate unit bounded by granulite facies gneiss that experienced peak metamorphic temperatures above 800 °C. In bulk, the deposit is dominated by wollastonite, but varied proportions of garnet, diopside, quartz, calcite, and vesuvianite are also present. Mineral chemistry variations across the deposit are minor, and the absence of inclusions indicates textural and chemical equilibrium. The wollastonite-bearing rocks have unusually low mineral δ18O values: –0.6 to +2.2‰ for garnet, –0.2 to +2. 6‰ for clinopyroxene, and –0.2 to +0.4‰ for wollastonite. Calcite δ18O values range from 6.8 to 11. 8‰ and δ13C values from –6.4 to –3.2‰. Calcite δ18O values are unusually low for calc-silicate rocks, but Δcalcite-garnet values from 3 to 12‰ indicate O-isotope disequilibrium between calcite and the silicate minerals. Garnet-biotite metapelitic and diopside gneisses have unexpectedly low δ18O values (<7‰). The approach to O-isotope equilibrium displayed by coexisting silicate minerals, and low mineral δ18O values in calc-silicate and metapelite and metapsammite gneisses, is consistent with low δ18O values being acquired before peak metamorphism. Low δ18O values in the minerals of the calc-silicate rocks require interaction with external fluid at high water/rock ratio. We suggest that the deposit represents a metamorphosed skarn that developed at the contact between the original carbonate rocks and intruding felsic magmas.

Multistage Evolution of Proterozoic Crust of East Antarctica by the Example of the Filla Terrane (Rauer Islands): New Geological and Isotope Data

—The paper presents new data on the Rauer Islands, one of the unique objects of the East Antarctic Shield. The interest in this area is triggered by its complex geologic structure, including both Archean and Proterozoic fragments of the Earth’s crust, and by its multiphase formation. A detailed scheme of the geologic structure of the area is proposed, new petrologic complexes are revealed, and the stages of tectonomagmatic activity at ~1400–1320 Ma and 1150 Ma are reliably dated. This serves as a factual basis for comparison the study area with other regions of East Antarctica. Based on the geological and isotope data obtained, the Meso–Neoproterozoic Filla Terrane in the area of the Rauer Islands is recognized. It is composed of metamorphic and primarily intrusive rocks, whose protoliths formed in the time interval 1400–950 Ma. Three periods of tectonothermal activity have been established in the Filla Terrane: Mid-Mesoproterozoic (1400–1320 Ma), Meso–Neoproterozoic (1150–886 Ma), and early Cambrian (536–504 Ma). The first period is the formation time of Mesoproterozoic crust, and it is time-correlated with the tectonogenesis phase in the adjacent Rayner province. The second period corresponds to the later phase of tectonothermal activity in the Rayner province. In the Filla Terrane, this period can be divided into two intervals, 1150–1100 Ma and 1010–886 Ma. The former interval is treated as intense crustal growth in the course of granitoid and mantle magmatism. The latter interval is a period of tectonothermal processes accompanied by intense deformations, high-temperature metamorphism, and intrusion of porphyritic granitoids. Apparently, the gap between the first and the second intervals is the time of deposition of the sedimentary protolith of paragneisses, which, together with the surrounding rocks, underwent high-temperature metamorphism and deformations at 950–914 Ma. The synchronous evolution of the Archean block and the Filla Terrane began at least within 1100–1000 Ma. The youngest, early Cambrian period of tectonic activity coincides with the development of local low-temperature mylonite zones and the intrusion of synkinematic pegmatite veins. Thus, the tectonothermal evolution of the Filla Terrane includes almost the same main phases of crustal growth and transformation as the Rayner province. This indicates that the Filla Terrane is a fragment of the Rayner province, which accreted to the Archean terrane at least in the late Mesoproterozoic.

Unraveling potential biases in U-Pb detrital zircon record induced by high-temperature metamorphism (> 850 ºC)

<p>Detrital zircon records are prone to several sources of bias that can compromise sediment provenance investigations based on U-Pb ages. High-temperature metamorphism (>850 ºC) is herewith addressed as a natural cause of bias since U-Pb zircon data from rocks submitted to these extreme, often prolonged conditions, frequently display protracted apparent concordant geochronological U-Pb records. The resulting spectrum can originate from disturbance of the primary U-Pb zircon system, likewise from subsequent recrystallization and crystallization processes during multiple and/or prolonged metamorphic events. Consequently, a high-grade metamorphosed igneous rock can exhibit a zircon age spectrum similar to that produced by polymict sedimentary rocks, thereby inducing provenance misinterpretations if this rock becomes a source for a sediment. A polymict sedimentary source that undergoes such high temperatures could potentially generate an even more intricate spectrum. Archean, Neoproterozoic and Paleozoic metamorphic rocks from the literature, dated by different techniques (SIMS and LA-ICP-MS), are employed as examples to demonstrate the resulting complications.  The compilation shows that (1) high-temperature metamorphism may generate age peaks of unclear or lacking geological meaning, and (2) the interpretation of detrital zircon age spectra depends on the timing of the metamorphic event (pre- or post-depositional). When high-temperature metamorphic rocks are eroded in uplifted areas, the youngest population of a detrital spectrum represents the maximum depositional age through metamorphic zircon from the source. If a sedimentary succession was subjected to high-temperature metamorphic conditions after deposition, its youngest zircon population more likely records the metamorphism, and the maximum depositional age, as well as older sources cannot be directly accessed. To evaluate the presence of high-temperature metamorphism-related bias in a given detrital zircon sample, we suggest a workflow for data acquisition and interpretation, combining a multi-proxy approach with: in situ U-Pb dating coupled with Hf analyses to retrieve the isotopic composition of the sources, and the integration of a petrochronological investigation to typify fingerprints of the (ultra)high-temperature metamorphic event.</p>

The evolution of the Sesia Zone (Western Alps) from Carboniferous to Cretaceous: insights from zircon and allanite geochronology

Microscale dating of distinct domains in minerals that contain relics of multiple metamorphic events is a key tool to characterize the polyphase evolution of complex metamorphic terranes. Zircon and allanite from five metasediments and five metaintrusive high-pressure (HP) rocks from the Eclogite Micaschist Complex of the Sesia Zone were dated by SIMS and LA-ICP-MS. In the metasediments, zircon systematically preserves detrital cores and one or two metamorphic overgrowths. An early Permian age is obtained for the first zircon rim in metasediments from the localities of Malone Valley, Chiusella Valley and Monte Mucrone (292 ± 11, 278.8 ± 3.6 and 285.9 ± 2.9 Ma, respectively). In the Malone Valley and Monte Mucrone samples, the early Permian ages are attributed to high-temperature metamorphism and coincide with the crystallization ages of associated mafic and felsic intrusions. This implies that magmatism and metamorphism were coeval and associated to the same tectono-metamorphic extensional event. In the Malone Valley, allanite from a metasediment is dated at 241.1 ± 6.1 Ma and this age is tentatively attributed to a metasomatic/metamorphic event during Permo-Triassic extension. Outer zircon rims with a late Cretaceous age (67.4 ± 1.9 Ma) are found only in the micaschist from Monte Mucrone. In metagabbro of the Ivozio Complex, zircon cores yield an intrusive age for the protolith of 340.7 ± 6.8 Ma, whereas Alpine allanite are dated at 62.9 ± 4.2 and 55.3 ± 7.3 Ma. The Cretaceous ages constrain the timing of the HP metamorphic stage. The presence of zircon overgrowth only in the central area of the Eclogite Micaschist Complex is attributed to local factors such as (1) multiple fluid pulses at HP that locally enhanced zircon dissolution and recrystallization, and (2) slightly higher temperatures reached in this area during HP metamorphism.

Geochemical Characterization of Zircon in Fyfe Hills of the Napier Complex, East Antarctica

Ultra-high temperature (UHT) metamorphism plays an essential role in the development and stabilization of continents through accretionary and collisional orogenesis. The Napier Complex, East Antarctica, preserves UHT metamorphism, and the timing is still debated. U–Pb zircon geochronology integrated with rare earth element (REE) and oxygen isotope was applied to a garnet-bearing quartzo-feldspathic gneiss to confirm the timing of UHT metamorphism in Fyfe Hills in the western part of the Napier Complex. The zircons are analyzed using a sensitive high-resolution ion microprobe (SHRIMP). The cathodoluminescence observation and U–Pb ages allowed us to classify the analytical domains into three types: inherited domains (Group I), metamorphic domains (Group II), and U–Pb system disturbed domains (Group III). The REE patterns of Group II are characterized by a weak fractionation between the middle REE and heavy REE, which reinforces the above classification. The 207Pb*/206Pb* ages of Group II have an age peak at 2501 Ma, therefore, the gneiss experienced high temperature metamorphism at 2501 Ma. δ18O of zircons are homogeneous among the three groups (5.53 ± 0.11‰, 5.51 ± 0.14‰, and 5.53 ± 0.23‰), which suggests re-equilibration of oxygen isotopes after metamorphism at ca. 2501Ma under dry UHT conditions.

Supplemental Material: Early Eocene high-flux magmatism and concurrent high-temperature metamorphism in the Gangdese belt, southern Tibet

These materials include mineral and whole-rock geochemical, zircon U-Pb dating, and monazite U-Th/Pb dating data from the Nymo intrusive complex of the Gangdese belt, as well as the compiled whole-rock geochemical data of early Eocene plutonic rocks from the Gangdese belt, southern Tibet.

Supplemental Material: Early Eocene high-flux magmatism and concurrent high-temperature metamorphism in the Gangdese belt, southern Tibet

These materials include mineral and whole-rock geochemical, zircon U-Pb dating, and monazite U-Th/Pb dating data from the Nymo intrusive complex of the Gangdese belt, as well as the compiled whole-rock geochemical data of early Eocene plutonic rocks from the Gangdese belt, southern Tibet.