Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

Crustal metamorphism under extreme pressure-temperature conditions produces characteristic ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) mineral assemblages at convergent plate boundaries. The formation and evolution of these assemblages have important implications, not only for the generation and differentiation of continental crust through the operation of plate tectonics, but also for mountain building along both converging and con- verged plate boundaries. In principle, extreme metamorphic products can be linked to their lower-grade counterparts in the same metamorphic facies series. They range from UHP through high-pressure (HP) eclogite facies to blueschist facies at low thermal gradients and from UHT through high-temperature (HT) granulite facies to amphibolite facies at high thermal gradients. The former is produced by low-temperature/pressure (T/P ) Alpine-type metamorphism during compressional heating in active subduction zones, whereas the latter is generated by high-T/P Buchan-type metamorphism during extensional heating in rifting zones. The thermal gradient of crustal metamorphism at convergent plate boundaries changes in both time and space, with low-T/P ratios in the compressional regime during subduction but high-T/P ratios in the extensional regime during rifting. In particular, bimodal metamorphism, one colder and the other hotter, would develop one after the other at convergent plate boundaries. The first is caused by lithospheric subduction at lower thermal gradients and thus proceeds in the compressional stage of convergent plate boundaries; the second is caused by lithospheric rifting at higher thermal gradients and thus proceeds in the extensional stage of convergent plate boundaries. In this regard, bimodal metamorphism is primarily dictated by changes in both the thermal state and the dynamic regime along plate boundaries. As a consequence, supercontinent assembly is associated with compressional metamorphism during continental collision, whereas supercontinent breakup is associated with extensional metamorphism during active rifting. Nevertheless, aborted rifts are common at convergent plate boundaries, indicating thinning of the previously thickened lithosphere during the attempted breakup of supercontinents in the history of Earth. Therefore, extreme metamorphism has great bearing not only on reworking of accretionary and collisional orogens for mountain building in continental interiors, but also on supercontinent dynamics in the Wilson cycle.

Supplemental Material: A metasedimentary source of gold in Archean orogenic gold deposits

Compositions of metasedimentary samples, and Table S2 (geochemical compositions of Pontiac, Porcupine and Cadillac samples collected in this study including sample locations, grid references and metamorphic facies grouping; black shale database including Au, As, and Sb data and sample name; and Mann-Whitney U-test results).<br>

Supplemental Material: A metasedimentary source of gold in Archean orogenic gold deposits

Compositions of metasedimentary samples, and Table S2 (geochemical compositions of Pontiac, Porcupine and Cadillac samples collected in this study including sample locations, grid references and metamorphic facies grouping; black shale database including Au, As, and Sb data and sample name; and Mann-Whitney U-test results).<br>

Experimental investigation of glaucophane rheology through shear and axial compression Griggs apparatus experiments on hot-pressed aggregates

&lt;p&gt;Constraining the rheological properties of glaucophane is critical to understanding subduction zone rheology. Based on the rock record, glaucophane is a major constituent mineral associated with subducted mafic oceanic crust at blueschist metamorphic facies. No flow law describing the crystal-plastic deformation of this mineral has been developed. Previous experimental work involving glaucophane focused on the deformation of natural polyphase rocks with an emphasis on seismic anisotropy. Here we focus on experiments intended to activate crystal-plastic deformation mechanisms in glaucophane using a monophase aggregate powder separated from natural samples from Syros Island, Greece. We are conducting general shear and axial compression experiments in a Griggs apparatus using temperatures of 600-800&amp;#176;C, pressures of 1 GPa and shear strain rates between 10&lt;sup&gt;-5&lt;/sup&gt;-10&lt;sup&gt;-6&lt;/sup&gt;. Our first experiment was in a general shear orientation at 700&amp;#176;C, 1 GPa, and a shear strain rate of 1.18x10&lt;sup&gt;-5&lt;/sup&gt;. This experiment had a ~80% modal abundance of glaucophane and appears to have been dominated by brittle deformation. After the first experiment, we decided to produce a purer glaucophane aggregate powder containing ~95% glaucophane with ~5% other phases and are finishing mineral separation at the time of submission. We will present early mechanical and microstructural data from experiments with the aim of developing a glaucophane flow law. Our results will also be compared to ongoing experiments focused on the viscous properties of experimentally deformed natural aggregates (see abstract in this conference by Tokle et al.).&lt;/p&gt;&lt;p&gt;&lt;br&gt;&lt;br&gt;&lt;/p&gt;

Deciphering the coupling between tectonic and metamorphic processes in the Monte Rosa nappe (Western Alps)

&lt;p&gt;Our refined ability to estimate metamorphic conditions incurred by rocks has increased our understanding of the dynamic earth. Calculating pressure (P), temperature (T) and time (t) histories of these rocks is vital for reconstructing tectonic movements within subduction zones. However, large disparities in peak P within a structurally coherent tectonic unit poses difficulties when attempting to resolve a tectono-metamorphic history, if a depth dependant lithostatic P is assumed. However, what is clear is that pressure, or mean stress, in a rock cannot exactly be lithostatic during an orogeny due to differential stress, required to drive rock deformation or to balance lateral variations in gravitational potential energy. Deviations from lithostatic P is commonly termed tectonic pressure, and both its magnitude and impact on metamorphic reactions in disputed.&lt;/p&gt;&lt;p&gt;For the &amp;#8216;Queen of the Alps&amp;#8217; (the Monte Rosa massif), estimates for the maximum P recorded during Alpine orogenesis remain enigmatic. Large disparities in published estimates for peak P exist, ranging between 1.2 and 2.7 GPa. Moreover, the highest P estimates (2.2 - 2.7 GPa) are for rocks that comprise only a small percentage (&lt; 1%) of the total volume of the nappe (whiteschist bodies and eclogitic mafic boudins). We present newly discovered whiteschist lithologies that persistently exhibit higher P conditions (&lt;em&gt;c.&lt;/em&gt; 2.2 GPa) compared to metagranitic and metapelitic lithologies (&lt;em&gt;c.&lt;/em&gt; 1.4 - 1.6 GPa). Detailed mapping and structural analysis in these regions lack evidence for tectonic mixing. Therefore, we suggest that a &amp;#916;P 0.6 &amp;#177; 0.2 GPa during peak Alpine metamorphism could potentially represent tectonic pressure. Furthermore, we outline possible mechanisms that facilitate &amp;#916;P, namely mechanically- and/or reaction-induced. We present data from numerical models that exhibit significant &amp;#916;P (&lt;em&gt;c.&lt;/em&gt; 0.4 GPa) during a transient period of high differential stress prior to buckling and subsequent exhumation of viscous fold nappes, similar to exhumation mechanisms suggested for the Monte Rosa nappe. As well as this, we present new routines for calculating metamorphic facies distribution within numerical models of subduction zones that agree with natural distributions within orogens.&lt;/p&gt;&lt;p&gt;The maximum burial depth of the Monte Rosa unit was likely significantly less than 80 km (based on the lithostatic pressure assumption and minor volumes of whiteschist at &lt;em&gt;c.&lt;/em&gt; 2.2 GPa). Rather, the maximum burial depth of the Monte Rosa unit was presumably equal to or less than &lt;em&gt;c.&lt;/em&gt; 60 km, estimated from pressures of 1.4 - 1.6 GPa recorded frequently in metagranite and metapelitic lithologies. In order to understanding, more completely, a rocks metamorphic history, consideration of the interplay between tectonic and metamorphic processes should not be overlooked.&lt;/p&gt;

Ultra-High Pressure Metamorphism and Geochronology of Garnet Clinopyroxenite in the Paleozoic Dunhuang Orogenic Belt, Northwestern China

Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure–temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790–920 °C/28–41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389–370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried.

Mafite-ultramafites (Peridotitovy stream, Polar Urals): mineralogy, estimation of P–T-parameters of formation

The petrographic features and conditions of the formation of metamorphosed mafic-ultramafic rocks of the Ampelshor complex, which are part of a small massif in the southern part of the Marunke u block of the Polar Urals (Peridotitovyi stream) and are controlled by tectonic thrust faults, have been studied. Muscovite-albite-clinozoisite-amphibole rocks (metabasites) and pyroxene-amphibole-chlorite (metautramafic) rocks are described. Structural, textural, and mineralogical features of metamagmatites indicate the hypabyssal nature of metabasite and plutonic nature of metaultramaphite. In metabasites, primary minerals are represented by relics of amphiboles (pargasite, edenite), and in metaultramaphites — by olivine and, possibly, clinopyroxene (augite, diopside). Magnesiohastingsite and chermakite in metautramafite were formed either at the late magmatic or metamorphic stages of rock transformation. The studied rocks underwent low-temperature changes (t — 468–380 °C, P — 2–3 kbar), corresponding to the greenschist metamorphic facies.

Tectono-metamorphic evolution and significance of shear-zone lithologies in Akebono Rock, Lützow-Holm Complex, East Antarctica

We describe a major shear zone exposed at Akebono Rock and discuss its deformation and metamorphic history, with a view to providing a better understanding of the geological history of the Lützow-Holm Complex. Three deformation episodes are recognized: D1 produced open folds (F1), boudinage and a regional ductile foliation, whilst the related metamorphic facies is characterized by stable garnet. F1 folding is dominantly preserved in the eastern part of the study area. During D2, an isoclinal to tight asymmetric F2 folds developed mainly in the west part of the region, accompanied by an S2 shear, under biotite facies retrograde metamorphism. The D3 episode involved the formation of the major shear zone, characterized by mylonite and L-tectonite fabrics, which took place at ~610–660°C and 4–5 kbar. Large, sigmoidal garnet core domains have S-shaped inclusion trails, suggesting that syntectonic garnet growth occurred before the formation of the shear zone. Estimated P-T conditions suggest that the sigmoidal garnet-bearing amphibolite was recrystallized at a deeper crustal level and was brought to a higher level during the formation of the shear zone. Crustal-scale deformation involving syntectonic recrystallization and shearing of Akebono Rock is a key issue for reconsidering the evolution of the Lützow-Holm Complex.