Metamorphic Record Research Articles

Earth’s metamorphic secular evolution accessed by rutile

Rutile provides crucial insights into the evolution of plate tectonics and variations in tectonomagmatic modes over Earth’s history primarily because rutile predominantly tracks higher-pressure metamorphism (>1.0–1.2 GPa). Comparison of rutile age and compositional data with diverse proxies tracks the presence of low temperature/pressure (T/P) metamorphic conditions, enabling the tracing of plate tectonics onset and evolution. We present a new compilation of rutile U-Pb ages that lag zircon ages by approximately 50 Myr, potentially indicating cooling ages of metamorphic belts. The earliest rutile age peak (ca. 1.8 Ga) aligns with the oldest evidence for low T/P metamorphic conditions. This coincides with a shift in the T/P distribution of rutile whereas the T/P of Archean rutile forms a tight normal distribution. In the Paleoproterozoic, the rutile T/P distribution is skewed to lower T/P, following the broader pattern of the full metamorphic record. The degree of lower T/P skew increases through time until the lower T/P population dominates the rutile record from the Neoproterozoic to the present. These data further highlight shifts in dominant metamorphic modes that can be tied to the evolution of the tectonic system, with early Archean metamorphism being associated with plume-driven vertical tectonics or hot, shallow subduction and post-Archean metamorphism being associated with progressive less plume-driven vertical tectonics and colder, steeper subduction. Although the frequency of rutile is lower during the Mesoproterozoic, the rutile T/P during this time follows the progressive skewing pattern from the Paleoproterozoic to the present and is likely inconsistent with a phase of interrupted tectonics characterized by stagnant lid.

Comment on Rocha et al. (2024): “The passive margin of the southern São Francisco paleocontinent, metamorphic record and implications for the assembly of West Gondwana: Evidence from the Lima Duarte Nappe, Ribeira Orogen (SE Brazil)”, Precambrian Research 404, 107338

Comment on Rocha et al. (2024): “The passive margin of the southern São Francisco paleocontinent, metamorphic record and implications for the assembly of West Gondwana: Evidence from the Lima Duarte Nappe, Ribeira Orogen (SE Brazil)”, Precambrian Research 404, 107338

Thinning and Heating of Laramide Continental Lower Crust Recorded by Zircon Petrochronology

AbstractZircon grains from the metasedimentary lower crust of the Rio Grande Rift, New Mexico, preserve a metamorphic record of the transition from Laramide compression to Eocene extension. Zircon U‐Pb isotopes and trace‐element concentrations from five two‐pyroxene metaigneous granulite xenoliths define discrete populations: older zircon cores (∼15–50 Ma) that are depleted in heavy rare‐earth elements (HREE) but Ti‐rich, and younger zircon rims (∼3–15 Ma) with elevated HREE and lower Ti concentrations. Coupled phase equilibria and garnet‐melt‐zircon trace‐element partitioning calculations show that the older zircon cores equilibrated in thick (>40 km), hot (800–900°C), garnet‐bearing lower crust during the cessation of compression at the end of the Laramide orogeny. Zircon rim domains equilibrated at lower pressures, consistent with >9 km of thinning of the lower crust. Thermal‐kinematic calculations show that these pressure‐temperature‐time constraints require thinning of the lithospheric mantle prior to and during regional Cenozoic extension. Convective erosion of the mantle lithosphere over tens of millions of years, possibly facilitated by dynamics of the Farallon slab, provides a mechanism to facilitate lower crustal heating and extension.

Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin

In this contribution, we investigate the ultra-high pressure metamorphism (UHPM), and subsequent pressure–temperature-time (P-T-t) exhumation path recorded by granulites (rutile-kyanite-garnet-Kfeldspar and rutile-titanite-hornblende-garnet gneisses) of a coherent thrust package (Três Pontas-Varginha Nappe) of the southernmost edge of the Brasília Orogen. The Brasília Orogen, along with Northern Borborema Province, Dahomey, and Hoggar Belts, comprise a widespread linear system of orogens developed during the West Gondwana assembly, recording the diachronic closure of the Tonian-Ediacaran long-lived Goiás-Pharusian Ocean. Optical petrography, Raman spectroscopy, mineral chemistry, Ti-in-zircon and Zr-in-rutile thermometry, and U-Pb dating (LA-ICP-MS) of zircon, monazite, and rutile were combined to identify UHP minerals and textures and to track the decompression/exhumation path. Garnet porphyroblasts preserve UHPM chemical domains and are often radially fractured around rounded quartz inclusions. Remnants of microcoesite were detected by Raman spectroscopy within these inclusions, identified by the diagnostic bands at approximately 170 cm−1, 270 cm−1, and 520 cm−1. Coesite remnants were identified across the nappe stack along with 60° angle-oriented rutile needle inclusions in garnet, which suggests exsolution from Ti-bearing UHP garnets. Soccer ball zircon crystals of Ky-bearing granulites, with flat HREE pattern and negative Eu anomaly, yielded a U-Pb age of around 620 Ma. This age is interpreted as the record of the granulite facies metamorphism (750–805 °C and 15 kbar) post-dating the UHPM from the subduction channel. Zircon and monazite record exhumation under high temperatures around 595–590 Ma, and rutile crystals record the final stages of the decompression path from 590 to 585 Ma. Monazite crystallization occurred until 570 Ma. The new data combined with previously reported metamorphic paths, record a time interval of about 25 m.y. for the exhumation of the Tres Pontas-Varginha Nappe under subsolidus conditions. Our findings represent one of the oldest reports of Neoproterozoic coesite-forming UHPM recorded in metasedimentary rocks buried to mantle depths in the subduction of the passive continental margin.

Metamorphic turnover at 2 Ga related to two-stage assembly of Columbia

Understanding the stabilization of cratons and how this is related to the onset of plate-tectonics is among the most important questions in geoscience. The assembly of Earth’s first supercontinent Columbia represents the first lines of evidence for a global subduction network, when the oldest, deep subduction-related rocks have been reported. We combine the low-, intermediate- and high-T/P global metamorphic record with the two-stage assembly of the Nuna-Columbia supercontinent to address the significance of the oldest “cold” rocks (low-T/P) and the related emergence of bimodal metamorphic belts. For this purpose, we analyse two examples from Laurentia (including Greenland) and Australia between 2.0–1.8 Ga and 1.8–1.6 Ga. Two main observations are: (i) a first-stage (2.0–1.8 Ga) amalgamation of the megacontinent Nuna (precursor to Columbia) is characterized by bimodal metamorphism along major mobile belts suturing the megacontinent’s center. In contrast, a second-stage (1.8–1.6 Ga) is dominated by the formation of soft collisional orogens during the final Columbia supercontinent assembly, recording intermediate- to high-T/P metamorphism; (ii) the metamorphic signature of the two assembly stages, featuring low- and intermediate-T/P rocks during Nuna assembly followed by their near absence during Columbia amalgamation, contrasts with the thermobaric ratios recorded by the Phanerozoic Gondwana-Pangea assembly, where intermediate and low-T/P rocks dominated the final stage of Pangea amalgamation. This discrepancy may signify substantial changes in intraplate metamorphism and minor rearrangements during Columbia assembly compared to major continent–continent collisions, such as the Appalachian-Variscan Orogen as well as production and fast exhumation of high- to ultra-high-pressure rocks during the assembly of the supercontinent Pangea. Furthermore, the variation of thermobaric ratios aligns with the concept of a two-stage mega-supercontinent formation, emphasizing differences between the potentially oldest and youngest supercontinent cycles.

The passive margin of the southern São Francisco paleocontinent, metamorphic record and implications for the assembly of West Gondwana: Evidence from the Lima Duarte Nappe, Ribeira Orogen (SE Brazil)

In this contribution, we report new P-T (pressure–temperature) and U-Pb zircon geochronological data from the Lima Duarte Nappe and basement rocks from the Mantiqueira Complex, Central Ribeira Orogen, southeastern Brazil, highlighting their relevance to the understanding of the West Gondwana assembly. The lower metasedimentary sequence of the Lima Duarte Nappe records a passive margin environment, with a majority of Paleoproterozoic detrital zircon grains, with probable main source rocks located in the São Francisco paleocontinent. The upper foreland sequence of the Lima Duarte Nappe has a prevalence of Tonian-Cryogenian detrital zircon grains, absent in the lower passive margin sequence and source rocks were most likely in a magmatic arc developed during the convergence between the São Francisco and the Paranapanema paleoplates. Zircon rims from the Lima Duarte passive margin and foreland sequences, and zircon grains from an amphibolite of the Lima Duarte Nappe record granulite facies metamorphism at ca. 560–590 Ma (Th/U ratio < 0.1). Similar ages are recorded by CL-dark zircon rims and overgrowths in orthogneisses of the Mantiqueira Complex and are interpreted to reflect basement reworking. Metapelitic metatexites from the foreland sequence record granulite facies peak metamorphism at ∼ 780–800 °C and 8–9.5 kbar in the sillimanite-rutile stability field, constrained by phase equilibrium modeling. Rutile inclusions in garnet record minimum peak temperatures of ∼ 750 °C. The retrograde mineral assemblage in the matrix is marked by the disappearance of rutile and the presence of ilmenite and mineral compositions re-equilibrated at upper amphibolite facies. Late Tonian (ca. 850–750 Ma) metamafic rocks within the Mantiqueira and Juiz de Fora complexes might record an extensional intraplate mafic magmatism episode that culminated with the establishment of the passive margin at the south of the São Francisco paleocontinent. The Lima Duarte metasedimentary sequence along with the Canastra and Carrancas groups (western and southern portions of the Brasília Orogen, respectively) represent a Tonian proximal passive margin environment, developed at the margin of the São Francisco paleocontinent. The Lima Duarte Nappe metasedimentary units and the orthogneisses of the Mantiqueira Complex record Ediacaran syn-collisional metamorphism related to the Araçuaí-Ribeira Orogenic System.

Ultrahigh thermal gradient granulites in the Narryer Terrane, Yilgarn Craton, Western Australia, provide a window into the composition and formation of Archean lower crust

AbstractGranulites from the Narryer Terrane in the northern Yilgarn Craton, Australia, record evidence for high to ultrahigh thermal gradients during the Meso–Neoarchean. U–Pb zircon ages reflect a complex history of high‐grade, prolonged and poly‐phase metamorphism, with evidence for several thermal pulses at ca. 2745–2725, ca. 2690–2660, and ca. 2650–2610 Ma. Forward phase equilibrium modeling on rocks with varying bulk compositions and mineral assemblages suggests that peak temperatures reached 880–920°C at pressures of 5.5–6 kbar at ca. 2690–2665 Ma, followed by near‐isobaric cooling. These new P–T results also indicate that these rocks experienced some of the hottest thermal gradient regimes in the metamorphic record (≥150°C/kbar). Based on P–T models, U–Pb ages, and geochemical constraints, our data suggest that the geodynamic setting for the formation of this unusual thermal regime is ultimately tied to cratonization of the Yilgarn Craton. Previous models have inferred that ultrahigh thermal gradients and coeval large‐scale anatexis in the Narryer Terrane were primarily generated by mantle‐driven processes, despite most of the lithological, isotopic, and geochemical observations being at odds with the expected geological expression of large‐scale mantle upwelling. We re‐evaluate the mechanisms for high‐grade metamorphism in the Narryer terrane and propose that long‐lived high crustal temperatures between ca. 2690 Ma and 2610 Ma were instead facilitated by elevated radiogenic heat production in thickened, highly differentiated ancient crust. Mantle‐derived magma input and new crustal addition may not be the only drivers for high‐ to ultrahigh‐temperature metamorphism and stabilization of ancient crustal blocks.

Formation of multistage garnet grains by fragmentation and overgrowth constrained by microchemical and microstructural mapping

AbstractGarnet is an exceptionally useful mineral for reconstructing the evolution of metamorphic rocks that have experienced multiple tectonic or thermal events. Understanding how garnet crystallizes and its mechanical behaviour is important for establishing a petrological and temporal record of metamorphism and deformation and for recognizing multiple geologic stages within the growth history of an individual crystal. Here, we integrate fine‐scale microstructural (electron backscatter diffraction [EBSD]) and microchemical (Laser Ablation Inductively Coupled Plasma Mass Spectrometry [LA‐ICP‐MS] mapping) data obtained on a polycyclic garnet‐bearing micaschist from the Alpine belt. Results suggest that fragmentation of pre‐Alpine garnet porphyroblasts occurred during the late pre‐Alpine exhumation and/or the onset of the Alpine burial, such that the older pre‐Alpine garnet fragments were transported/redistributed during Alpine deformation and acted as nucleation sites for Alpine garnet growth. These processes produced a bimodal garnet size distribution (millimetre‐ and micrometre‐sized grains). Thermodynamic modelling indicates that Alpine garnet grew during the final stage of burial (from 1.9 GPa 480°C to 2.0 GPa 520°C) and early exhumation (down to 1.6 GPa 540°C) forming continuous idioblastic rims on and sealing fractures in pre‐Alpine garnet grains. We propose that fragmentation–overgrowth processes in polycyclic rocks, coupled with ductile deformation, may produce a bimodal garnet size distribution in response to fragmentation and re‐distribution of pre‐existing grains; these clasts can act as new nucleation sites during a subsequent orogenic cycle.

Identical metamorphic record in distinct petrochemical systems: Case study of microscopically interlayered garnet amphibolite and metapelite from the Danba dome, SW China

It is known that different metamorphic rocks may have different abilities in recording metamorphic processes. Here, we report microscopically interlayered garnet amphibolite and metapelite layers in a rock from the Danba dome in eastern Tibetan Plateau, SW China. Both rock types exhibit similar garnet zoning patterns, biotite and plagioclase compositions, as well as peak P-T conditions (7.3–8.0 kbar / 700–725 °C for metapelite and 7.1–8.4 kbar / 690–710 °C for garnet amphibolite, respectively), indicating a global thermodynamic equilibrium state and the same metamorphic record of the two distinct layers. However, medium-sized garnet exhibits homogeneous core compositions and biotite preserves lower Ti and Fe# [= Fe2+ / (Fe2+ + Mg)] at the lithological interface, possibly indicating more intense chemical homogenization at the metamorphic peak and retrograde modification at the interface caused by the possible presence of vast fluid. Sensitive high-resolution ion microprobe (SHRIMP) UPb dating of metamorphic zircon show a wide dispersion of ages (c. 200–155 Ma), overlapping previous age data reported for this region. These results indicate that differences in petrochemical compositions may not lead to significant distortion in the metamorphic records, including garnet zoning patterns and peak P-T conditions. This further suggests that metamorphic rocks originating from diverse protoliths, which exhibit varying peak P-T conditions within the same locality, could either be a result of the tectonic juxtaposition of distinct tectonic slices or the influence of varying fluid conditions.

Reinitiation of modern-style plate tectonics in the Early Neoproterozoic: Evidence from a ∼930 Ma blueschist-facies terrane in South China

When and how plate tectonics operated throughout Earth’s history remain a matter of debate. The absence of low-temperature/high-pressure metamorphic rocks during Earth’s middle age (∼1.8–0.8 Ga) leads to some arguing for a temporary shutdown or change of modern-style plate tectonics throughout the Mesoproterozoic and Early Neoproterozoic. Here we report the first discovery of ∼930 Ma blueschist-facies metamorphic rocks (Tianli Schist) from the Jiangnan Orogen, South China. Peak pressure–temperature conditions of ∼425–475 ℃ and ∼0.8–1.1 GPa recorded by these units represent an average geothermal gradient of ∼14 ℃/km, characteristic of cold subduction zones. Available geologic and geochronologic constraints indicate that the Tianli Schist protolith was deposited in the southeast margin of the Yangtze Block and subsequently experienced blueschist-facies metamorphism during ocean closure at ∼930 Ma. Combined with the global metamorphic record, our results suggest that modern-style plate tectonics did not operate continuously since initiation in the Early Paleoproterozoic, but was inhibited in the Mesoproterozoic and restarted again at the end of Earth’s middle age.

Timescales and mechanisms of felsic lower continental crust formation: Insights from U-Pb geochronology of detrital zircon (Malenco Unit, eastern Central Alps)

Burial of supracrustal sedimentary material is one of the processes that contribute to the formation of the lower continental crust. We investigate the rate of this process in Permian lower continental crust from the Malenco Unit (eastern Central Alps) by constraining the age of protolith deposition and subsequent high-grade metamorphism of felsic, garnet-rich granulites. Detrital zircon cores, many of which are small (<50 μm), were dated by SIMS and yield concordant dates ranging from 387 ± 21 Ma (2σ) to 2380 ± 16 Ma, with the vast majority of the dates falling in the range 450–1000 Ma. For the seven cores in each sample that gave the youngest concordant dates, replicate analyses were carried out. None of these cores had all measured U-Pb dates agree within error, indicating significant disturbance by the Permian granulite facies metamorphism. A 206Pb/238U age of 424 ± 6 Ma for a core with two overlapping replicate analyses is the best available constraint on the maximum depositional age of the sedimentary protolith. This maximum depositional age also coincides with the first significant population in the probability density plot of detrital core dates, which is a double peak at ∼425 Ma and ∼450 Ma.Zircon detrital cores are separated from metamorphic overgrowths by a small seam of zircon that is riddled with tiny inclusions of quartz, biotite and muscovite. The first generation of metamorphic zircon rims has a 206Pb/238U age of 272.9 ± 2.8 Ma, displays steep heavy rare earth element (HREE) patterns and a moderate negative Eu-anomaly, and returns Ti-in-zircon temperatures of 740–780 °C. The second generation of metamorphic rims has a 206Pb/238U age of 263.8 ± 2.6 Ma, a flat HREE pattern and a pronounced negative Eu-anomaly, and gave Ti-in-zircon temperatures of 780–810 °C. Collectively these observations indicate that the first zircon rim formed on the prograde path at the onset of partial melting where muscovite was present but the rocks contained little garnet and no K-feldspar. Therefore, the metapelites resided at lower amphibolite facies, subsolidus conditions up until the intrusion of a Permian gabbro, which was emplaced shortly after the formation of the first generation of metamorphic zircon and caused heating to granulite facies conditions and widespread partial melting. There is no record of high-pressure metamorphism preceding granulite facies conditions, indicating that felsic lower crust in the Malenco unit was not formed by relamination.The Malenco unit was situated at the north-eastern active margin of Gondwana 420 Ma ago. Subduction-related accretion of sediments at lower crustal levels shortly after their deposition could explain the formation of this felsic lower crust that resided at subsolidus conditions for up to 100 My prior to Permian extension, gabbro intrusion and granulite facies metamorphism.

Ti-Bearing Minerals: from the Ocean Floor to Subduction and Back

Abstract Rutile, titanite, and ilmenite are the most common Ti-bearing minerals in metamorphic rocks. Experimental constraints have shown that titanite is stable at low-grade metamorphic conditions, rutile at high pressure (HP), and ilmenite at high temperature, low pressure (HT-LP) conditions. Yet, petrological evidence suggests that titanite can also be stable at low temperature, HP (LT-HP). This implies that both titanite and rutile can be used to develop proxies to track HP metamorphism, which can have interesting applications. In this study, we have investigated the natural occurrence and chemistry of Ti-bearing minerals in gabbroic rocks and basalts that record different degrees of metamorphism, including LP amphibole-bearing gabbros from the ocean floor (Mid-Atlantic and Indian ridge IODP LEGs) and from an obducted ophiolite (Chenaillet) and HP Alpine metagabbros and metabasalts, including blueschist and eclogite facies rocks from the Western Alps and Corsica. We have performed detailed petrography, Raman spectroscopy and analyzed major and trace elements mineral chemistry using EPMA and LA-ICPMS. We found that rutile is stable at low pressure (&amp;lt; 2 kbar) in ocean-floor amphibole-bearing gabbros, lower than experimental constraints had previously suggested. Rutile is also found in eclogitic metagabbros from the Western Alps and can be chemically distinguished from LP rutile. Blueschist metagabbros from the Western Alps and eclogitic metabasalts from Corsica have titanite stable instead of rutile. While the titanite to rutile transition is pressure- and temperature-dependent, we demonstrate how small variations in bulk-rock Ti/Ca and Ca/Al values within the NCKFMASHTO chemical system may shift their stabilities. High-pressure titanite from these metamafic rocks exhibits La depletion and low La/SmN values in comparison to titanite from amphibolite-facies mafic rocks. La/SmN or Nb together with Yb and V can be used to distinguish HP titanite from titanite formed under other P–T settings. These new systematics can be useful in studies using detrital Ti-bearing minerals to probe the HP metamorphic record through time.

Paralava and clinker from the Canadian Arctic: a record of combustion metamorphism dating back to the late Miocene

Outcrops with conspicuous reddish to yellow-colored clinker, blackish paralava, and blends of both with a breccia-like appearance occur across the Canadian Arctic. We examined such rocks on Ellesmere Island, Banks Island, and the Mackenzie Delta area. These rocks are a product from natural combustion of bituminous shale and low-rank coal seams in Cretaceous and Paleogene host sedimentary rocks, respectively. The main mineral phases of clinker and silicate paralava samples are comprised of quartz + hematite ± feldspars ± cristobalite (or tridymite) ± cordierite–sekaninaite ± clinopyroxene ± sillimanite ± glass. Slag-like iron oxide paralava (74–95 wt.% total Fe2O3) consisting of hematite ± magnetite ± clinopyroxene occur in Paleogene host sedimentary rocks, rich in siderite concretions. The whole-rock geochemical composition of clinker and silicate paralava shows similarities for samples from the same outcrop. Regional and local specific elemental enrichments are mainly inherited from the sedimentary protoliths, which are characterized by volcanogenic input (Paleocene sedimentary rocks) or oxygen-depleted depositional conditions (Upper Cretaceous bituminous sedimentary rocks). Spontaneous combustion could take place when the organic-rich sedimentary rocks become exposed to atmospheric oxygen. This process has occurred at least since the Messinian stage (Miocene) on Ellesmere Island (6.1 ± 0.2 Ma; 40Ar/39Ar incremental heating dating on whole-rock paralava) and continues until now. An active combustion process on scree from a coal seam and clastic Eureka Sound Group sedimentary rocks was observed on Ellesmere Island.

Late Neoproterozoic–Cambrian eclogites and high‐pressure granulites in the Central Qilian terrane (China) record the earliest subduction of Proto‐Tethyan Ocean in the eastern Tethysides

AbstractThe Central Qilian terrane (CQT) of the northern Tibetan Plateau played a key role in the tectonic evolution of the Proto‐Tethyan Ocean in the Tethysides, but its formation and tectonic attribution have been hotly debated. Here, we report the discovery of eclogites and HP mafic granulites in the Beidahe Complex of the western CQT. These occur as blocks of various sizes within a sequence of metavolcanic–sedimentary rocks, exhibiting typical a ‘block‐in‐matrix’ and thrust imbrication structure. The eclogite facies metamorphic rocks preserve distinct mineral assemblages and textures corresponding to prograde, peak, and retrograde metamorphism. By combining phase equilibrium modelling with SHRIMP and LA‐ICPMS U–Pb dating of metamorphic zircon, sphene, and rutile, the Late Neoproterozoic–Cambrian (c. 553–516 Ma) eclogite facies peak with low thermal gradients of 10–14°C/km, Cambrian (c. 515–506 Ma) post‐peak decompression and Ordovician (c. 495–455 Ma) cooling histories for these metabasic rocks have been restored. These constitute hairpin‐type clockwise pressure–temperature–time (P–T–t) paths depicting in detail the sequence of deep subduction and subsequent exhumation in Central Qilian during the Late Neoproterozoic to Early Palaeozoic. Our new findings suggest that the CQT represents a Japan‐type arc‐accretionary system that formed as a result of the North Qilian oceanic plate, one of the major branches of the Proto‐Tethys Ocean, being subducted southward. Eclogites in the Beidahe Complex in western CQT offer the earliest (c. 553 Ma) metamorphic record of subduction in the Qilian orogen, indicating that the North Qilian Ocean commenced subducting southward prior to the Late Neoproterozoic.

Corundum genesis at the Blue Jay Sapphire occurrence (British Columbia, Canada) as a record of metamorphism and partial melting in the Monashee Complex

Corundum genesis at the Blue Jay Sapphire occurrence (British Columbia, Canada) as a record of metamorphism and partial melting in the Monashee Complex

Turonian-coniacian definition of the Caribbean plate: tectonic and metamorphic record in the Median Belt, Dominican Republic

ABSTRACT Uplift and unroofing of Jurassic-Cretaceous, mantle and crust, arc- and plume-related rock units in the Median Belt of the Dominican Republic exposed basement rocks with a protracted record of tectono-thermal events delineating the evolution of the northern edge of the Caribbean plate. In this article we focus on crustal rock units in the north-eastern half of the Median Belt. First 40Ar/39Ar dating of metamorphic ferri-winchite (86.4 ± 2.5 Ma; crystallization date) and albite (82.3 ± 5.8 and 79.8 ± 1.6 Ma; cooling dates) in metabasites of boninitic photolith from the Aptian-Albian Maimón Formation in the Ozama shear zone points to a tectono-metamorphic event in the Upper Cretaceous. Beltwide, this event caused syn-metamorphic N- to NNE-directed, simple-shear dominated, mylonitic and phyllonitic deformation of the Maimón Formation at peak metamorphic conditions of 8.2 kbar and 380°C. Such conditions are consistent with subduction of a coherent portion of forearc (represented by the Maimón Formation) to depths of ~25-29 km. The tectono-metamorphic event dated here overlaps with the inception of Turonian-Coniacian SW-dipping subduction and metamorphic sole formation in a back-arc position recorded in the Moa-Baracoa ophiolite complex in neighboring Eastern Cuba. Contemporaneity between the subduction inception of forearc and back-arc portions of the Caribbean arc and the main pulse of plume activity recorded in the Caribbean Large Igneous Province (CLIP) suggests that plume activity promoted general plate instability leading to a regional-scale plate reorganization. This mantle-plume-induced plate margin reorganization was coeval with the inception of the NE-dipping subduction of the Farallón plate beneath Central America leading to the definition of the Caribbean Plate by double-verging subduction zones along its northern and southwestern margins.

Earth's anomalous middle-age magmatism driven by plate slowdown

The mid-Proterozoic or "boring billion" exhibited extremely stable environmental conditions, with little change in atmospheric oxygen levels, and mildly oxygenated shallow oceans. A limited number of passive margins with extremely long lifespans are observed from this time, suggesting that subdued tectonic activity—a plate slowdown—was the underlying reason for the environmental stability. However, the Proterozoic also has a unique magmatic and metamorphic record; massif-type anorthosites and anorogenic Rapakivi granites are largely confined to this period and the temperature/pressure (thermobaric ratio) of granulite facies metamorphism peaked at over 1500 °C/GPa during the Mesoproterozoic. Here, we develop a method of calculating plate velocities from the passive margin record, benchmarked against Phanerozoic tectonic velocities. We then extend this approach to geological observations from the Proterozoic, and provide the first quantitative constraints on Proterozoic plate velocities that substantiate the postulated slowdown. Using mantle evolution models, we calculate the consequences of this slowdown for mantle temperatures, magmatic regimes and metamorphic conditions in the crust. We show that higher mantle temperatures in the Proterozoic would have resulted in a larger proportion of intrusive magmatism, with mantle-derived melts emplaced at the Moho or into the lower crust, enabling the production of anorthosites and Rapakivi granites, and giving rise to extreme thermobaric ratios of crustal metamorphism when plate velocities were slowest.

Laser-ablation Lu-Hf dating reveals Laurentian garnet in subducted rocks from southern Australia

Abstract Garnet is a fundamental expression of metamorphism and one of the most important minerals used to constrain the thermal conditions of the crust. We used innovative in situ laser-ablation ICP-MS/MS Lu-Hf geochronology to demonstrate that garnet in metapelitic rocks enclosing Cambrian eclogite in southern Australia formed during Laurentian Mesoproterozoic metamorphism. Garnet porphyroblasts in amphibolite-facies metapelitic rocks yielded Lu-Hf ages between 1286 ± 58 Ma and 1241 ± 16 Ma, revealing a record of older metamorphism that was partially obscured by metamorphic overprinting during ca. 510 Ma Cambrian subduction along the East Gondwana margin. Existing detrital zircon age data indicate the protoliths to the southern Australian metapelitic rocks were sourced from western Laurentia. We propose that the metapelitic rocks of southern Australia represent a fragment of western Laurentian crust, which was separated from Laurentia in the Neoproterozoic and incorporated into the East Gondwana subduction system during the Cambrian. The ability to obtain Lu-Hf isotopic data from garnet at acquisition rates comparable to those for U-Pb analysis of detrital zircon means, for the first time, the metamorphic parentage of rocks as expressed by garnet can be efficiently accessed to assist paleogeographic reconstructions.

A diachronous record of metamorphism in metapelites of the Western Gneiss Region, Norway

AbstractIn this study, data from garnet‐kyanite metapelites in ultrahigh‐pressure (UHP) domains of the Western Gneiss Region (WGR), Norway, are presented. U–Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and pressure–temperature (P–T) conditions were calculated using mineral equilibria forward modelling and Zr‐in‐rutile thermometry. Garnet‐kyanite gneiss from Ulsteinvik record a prograde evolution passing through ~690–710°C and ~9–11 kbar. Zircon and rutile age and thermometry data indicate these prograde conditions significantly pre‐date Silurian UHP subduction in the WGR and are interpreted to record early Caledonian subduction of continental‐derived allochthons. Minimum peak conditions in the Ulsteinvik metapelite occur at ~28 kbar, constrained by an inferred garnet+kyanite+omphacite+muscovite+rutile+coesite+H2O assemblage. The retrograde evolution passed through ~740°C and ~7 kbar, first recorded by the destruction of omphacite and followed by the partial replacement of kyanite and garnet by cordierite and spinel. Garnet‐kyanite metapelite from the diamond‐bearing Fjørtoft outcrop documents a polymetamorphic history, with garnet forming during the late Mesoproterozoic and limited preservation of high‐pressure Caledonian assemblages. Similar to the Ulsteinvik metapelite, zircon and rutile age data from the Fjørtoft metapelite also record pre‐Scandian Caledonian ages. Two potential tectonic scenarios are possible: (1) The samples were exhumed at different times during pre‐Scandian subduction of the Blåhø nappe, or (2) the samples do not share a history in the same nappe complex, instead the Ulsteinvik metapelite is a constituent of the Seve‐Blåhø Nappe, whilst the Fjørtoft metapelite shares its history within a separate nappe complex.

A plate tectonic view from the top of the world

AbstractNew U–Pb calcite geochronological analysis of shear veins in specimens collected from the summit pyramid of Mount Everest demonstrate that upper crustal deformation recorded in those rocks is Himalayan in age and was ongoing at 45.0 ± 5.4 Ma. This deformation precedes movement along the adjacent Qomolangma detachment by ~30 Myrs and coincides with metamorphism in nearby, structurally deeper, sillimanite‐migmatite gneisses and eclogite. The coeval record of deformation and metamorphism across all crustal levels further coincides with detachment of the Indian oceanic lithospheric slab, the inferred “main” collision and final closure of the Tethys ocean in multi‐collisional models of the Himalaya, and Eocene reorganization of tectonic plates in the South Pacific. This new dataset confirms that the summit rocks from Mount Everest preserve not only a complex, multi‐deformational record of Himalayan orogenesis, but also large‐scale changes in plate tectonics.