Diachronous Redistribution of Hf and Nd Isotopes at the Crystal Scale—Consequences for the Isotopic Evolution of a Poly-Metamorphic Crustal Terrane

In metamorphic rocks, mineral species react over a range of pressure–temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure–temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history. Here, we present in-situ U–Pb/Sm–Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U–Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived from the oldest zircon cores and apatite U–Pb/Sm–Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the hand-specimen scale (≤~0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.

Rare allanite in granulitised eclogites constrains timing of eclogite to granulite facies transition in the Bhutan Himalaya

During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are determined by linking geochemical, geochronological and microstructural data from metamorphic rock-forming and accessory minerals. Exposures of lower orogenic crust provide important insights into orogenic evolution, but are rare in young continental collision belts such as the Himalaya. In NW Bhutan, eastern Himalaya, a high-grade metamorphic terrane provides a rare glimpse into the evolution and exhumation of the deep eastern Himalayan crust and a detailed case study for deciphering the rates and timescales of deep-crustal processes in orogenic settings. We have collected U-Pb isotope and trace element data from allanite, zircon and garnet from metabasite boudins exposed in the Masang Kang valley in NW Bhutan. Our observations and data suggest that allanite cores record growth under eclogite facies conditions (>17 kbar ~650°C) at ca. 19 Ma, zircon inner rims and garnet cores record growth during decompression under eclogite facies conditions at ca 17-15.5. Ma, and symplectitic allanite rims, garnet rims and zircon outer rims record growth under granulite facies conditions at ~9-6 kbar; >750°C at ca. 15-14.5 Ma. Allanite is generally considered unstable under granulite-facies conditions and we think that this is the first recorded example of such preservation, likely facilitated by rapid exhumation. Our new observations and petrochronological data show that the transition from eclogite to granulite facies conditions occurred within 4-5 Ma in the Eastern Himalaya. Our data indicate that the exhumation of lower crustal rocks across the Himalaya was diachronous and may have been facilitated by different tectonic mechanisms.

Garnet sand reveals rock recycling processes in the youngest exhumed high- and ultrahigh-pressure terrane on Earth

Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene–Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO2 inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO2 (cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on <10-My geologic timescales, including subduction of sedimentary protoliths to UHP conditions, rapid exhumation, surface uplift, and erosion. Detrital garnet geochemistry and inclusion suites from both modern sediments and stratigraphic sections can be used to decipher the petrologic evolution of plate boundary zones and reveal recycling processes throughout Earth’s history.

Regional Quartz Inclusion Barometry and Comparison with Conventional Thermobarometry and Intersecting Isopleths from the Connecticut Valley Trough, Vermont and Massachusetts, USA

A comparative analysis of Raman shifts of quartz inclusions in garnet was made along two traverses across the Connecticut Valley Trough (CVT) in western New England, USA, to examine the regional trends of quartz inclusion in garnet (QuiG) Raman barometry pressure results and to compare this method with conventional thermobarometry and the method of intersecting garnet core isopleths. Overall, Raman shifts of quartz inclusions ranged from 1·2 to 3·5 cm–1 over all field areas and displayed a south to north decrease, matching the overall decrease in mapped metamorphic grade. Raman shifts of quartz inclusions typically did not show systematic variation with respect to their radial position within a garnet crystal, and indicate that garnet probably grew at nearly isothermal and isobaric pressure–temperature (P–T) conditions. The P–T conditions inferred from conventional thermobarometry were in the range of ∼500–575 °C and ∼7·4–10·3 kbar over the sample suite and are in good agreement with previous published thermobarometry throughout the CVT. These P–T results are broadly consistent with QuiG barometry and also suggest that garnet grew isothermally and isobarically at near peak P–T conditions. However, P–T conditions and P–T paths inferred using either garnet core thermobarometry or garnet core intersecting isopleths yield results that are internally inconsistent and generally disagree with the pressure results from QuiG barometry. Garnet core isopleth intersections consistently plotted between the nominal garnet-in curve on mineral assemblage diagrams and the P–T conditions constrained by QuiG isomekes for the majority of the sample suite. Additionally, most samples’ P–T results from QuiG barometry and rim thermobarometry show marked disagreement from those derived from garnet core thermobarometry, compared with the minority that showed agreement within uncertainty. Pressures calculated from QuiG barometry ranged from 8·5 to 9·5 kbar along the traverses in western Massachusetts (MA) and central Vermont (VT) and from 6·5 to 7·5 kbar in northern VT indicating an increase in peak burial of 3–6 km from north to south. Along the western end of the central VT traverse, there are differences in measured Raman shifts and inferred peak pressures of up to 1 kbar across the Richardson Memorial Contact (RMC), indicating a possible fault contact with minor post-peak metamorphic shortening of up to ∼3 km. In contrast, along an east–west traverse in the vicinity of the Goshen Dome, MA, there was little observed variation in Raman shifts across the contact. By contrast, QuiG barometry clearly indicates significant discontinuities in peak pressure east of the Strafford Dome in central VT. This supports the interpretation that post-peak metamorphic shortening was necessary to juxtapose upper staurolite–kyanite zone rocks next to lower garnet zone pelites. Overall, it is concluded that garnet core thermobarometry and garnet core isopleths may provide unreliable results for the P–T conditions of garnet nucleation and inferred P–T paths during garnet growth unless independently verified. The consistency of QuiG results with rim thermobarometry indicates that peak metamorphic conditions previously reported for the CVT using garnet rim thermobarometry are robust and that variation in QuiG barometry results is a valuable tool to analyze structural features within a metamorphic terrane.

Crystallographic orientation of ilmenite inclusions in amphibole – an electron backscatter diffraction study

Orientated ilmenite inclusions have been discovered in amphibole of hornblendite from the Zhujiapu area, Dabie ultra-high-pressure (UHP) metamorphic terrane, China. In order to characterize the crystallographic orientation relationships between ilmenite inclusions and amphibole host and reconstruct the mechanism of their formation, we present an electron backscatter diffraction (EBSD) analysis combined with energy dispersive spectroscopy (EDS) analysis and electron microprobe analysis (EPMA) for ilmenite inclusions and amphibole host. The inclusions can be subdivided into four groups: (1) 60.2% of ilmenites have the crystallographic orientation {0001}Ilm // {100}Amp, (101̅0)Ilm // {010}Amp, [112̅0]Ilm // <001> Amp and [112̅0]Ilm // <012 > Amp. (2) 16.5% of ilmenites have <0001> Ilm // <001> Amp, (101̅0)Ilm // {010}Amp, (112̅0)Ilm // {100}Amp and [3̅031]Ilm // <012> Amp. (3) 13.8% of ilmenites have <0001> Ilm // <012> Amp, (112̅0)Ilm // {100}Amp and [3̅031]Ilm // <001> Amp. (4) 9.5% of ilmenites have <0001> Ilm // [1̅12]Amp, (101̅0)Ilm // {201}Amp, [112̅0]Ilm // [1̅12]Amp and ${[11\overline {21} ]_{Ilm}}$// <010> Amp. By comparing the lattice relationship between ilmenite inclusions and amphibole hosts, it is shown that the frequency of the ilmenite inclusions in different groups is related to the lattice coherency and oxygen packing. Group-1 of the ilmenite inclusions was most likely be formed via a solid-state exsolution process by cooling of the hornblendite after the intrusion was emplaced. The other three groups of ilmenite inclusions were probably formed via reduction reaction in an open system. The formation temperature of the ilmenite inclusions is estimated by using the TiO2 solubility geothermeter in amphibole. The minimum formation temperature of the ilmenite inclusions is about 1025 °C, and the maximum formation temperature of the ilmenite inclusions is about 1126 °C.