Quartz-in-garnet barometry constraints on formation pressures of eclogites from the Franciscan Complex, California

Determining pressure and temperature variations between high-pressure/low-temperature (HP–LT) eclogite blocks is crucial for constraining end-member exhumation models; however, it has historically been challenging to constrain eclogite pressures due to the high variance associated with this bulk-rock composition. In this work, we utilize quartz-in-garnet elastic barometry to constrain formation pressures of eclogites from the northern (Junction School, Ring Mountain, Jenner Beach) and southern Franciscan Complex (Santa Catalina Island). Multiple eclogite blocks from Jenner Beach are analyzed, and a single eclogite from the other localities. By comparing garnet growth conditions from within a single outcrop and between distinct outcrops, we evaluate the local and regional spatial distribution of P conditions recorded by eclogites. We compare the mean, median, and max pressures between different garnet zones and eclogites. Pressures sometimes exhibit systematic changes across garnet zones; however, some eclogites exhibit no systematic pressure variations across garnet zones. Pressures from northern Franciscan eclogites range from $$\sim $$ ∼ 1.4–1.8 GPa, at an estimated temperature of 500 $$^{\circ }$$ ∘ C; pressures from the Catalina eclogite range from $$\sim $$ ∼ 1.2–1.5 GPa, at an estimated temperature of 650 $$^{\circ }$$ ∘ C. Mean and maximum pressures of different eclogites from the northern Franciscan exhibit negligible differences (< 0.1 GPa). The results are inconsistent with models that propose exhumation of metamorphic blocks from different structural levels, and suggest that now exposed HP–LT eclogites from the northern Franciscan Complex may represent rocks that were coherently underplated, and exhumed from similar structural levels.

Garnet Chemical Zoning Based Thermobarometry: Method Evaluation and Applications in the Menderes Massif, Western Turkey

The garnet chemical zoning method (GZM) is a reliable thermodynamic approach for forward modeling pressure-temperature (P-T) paths using observed garnet and bulk rock compositions. However, intracrystalline diffusion is known to compromise the integrity of GZM modeled garnet-growth P-T paths. For this reason, extracting reliable metamorphic estimates from garnet-bearing schists in the Central Menderes Massif (CMM), western Turkey, has been difficult. To evaluate the impact of diffusion on GZM, we simulate garnet growth and diffusion for an average metapelite using the program Theria_G. Modeled garnet compositions from four simulations are used to estimate P-T conditions and paths by GZM, which are compared against Theria_G specified P-T-t trajectories. Factors influencing results are heating/cooling rate, grain size, and peak T. At a maximum T of 610 °C, both undiffused and diffused garnet compositions returned estimates comparable to prescribed conditions regardless of heating/cooling rate. Diffused profiles from simulations reaching a maximum T of 670 °C also reproduced prescribed P-T paths if tectonism occurred at high heating/cooling rates (50 °C/my). From these insights and additional Theria_G simulation-derived observations for CMM garnets, we deduce that metamorphism in the region exceeded 650 °C and achieved a maximum burial P between 8–10 kbar prior to Cenozoic exhumation.

Reuniting the Ötztal Nappe: the tectonic evolution of the Schneeberg Complex

The Ötztal Nappe in the Eastern Alps is a thrust sheet of Variscan metamorphic basement rocks and their Mesozoic sediment cover. It has been argued that the main part of the Ötztal Nappe and its southeastern part, the Texel Complex, belong to two different Austroalpine nappe systems and are separated by a major tectonic contact. Different locations have been proposed for this boundary. We use microprobe mapping of garnet and structural field geology to test the hypothesis of such a tectonic separation. The Pre-Mesozoic rocks in the area include several lithotectonic units: Ötztal Complex s.str., Texel Complex, Laas Complex, Schneeberg Complex, and Schneeberg Frame Zone. With the exception of the Schneeberg Complex which contains only single-phased (Eoalpine, i.e. Late Cretaceous) garnet, all these units have two-phased garnet with Variscan cores and Eoalpine rims. The Schneeberg Complex represents Paleozoic sediments with only low-grade (sub-garnet-grade) Variscan metamorphism which was thrust over the other units and their Mesozoic cover (Brenner Mesozoic) during an early stage of the Eoalpine orogeny, before the peak of Eoalpine metamorphism and garnet growth. Folding of the thrust later modified the structural setting so that the Schneeberg Thrust was locally inverted and the Schneeberg Complex came to lie under the Ötztal Complex s.str. The hypothesized Ötztal/Texel boundaries of earlier authors either cut across undisturbed lithological layering or are unsupported by any structural evidence. Our results support the existence of one coherent Ötztal Nappe, including the Texel Complex, and showing a southeastward increase of Eoalpine metamorphism which resulted from southeastward subduction.

Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

<p>Metamorphic rocks have the potential to record in their mineral assemblages, mineral compositional zoning, and textures, information about geological changes and processes that occur during tectonic events. Interpretations of metamorphic pressure-temperature (P-T) records have traditionally relied on results of geothermobarometry studies, but that approach is not suitable in every case. Metamorphosed greywacke, which makes up ~95% of the New Zealand Southern Alps, has long proven problematic for traditional geothermobarometry because it develops intractable mineral compositions and/or assemblages, especially at relatively low temperature (greenschist facies) conditions. An alternative forward modelling approach using the computer program THERMOCALC was recently used to extract the first detailed P-T history (P-T path) from such previously intractably difficult "greyschist" rocks from a single site in the New Zealand Southern Alps. The present study is the first attempt to apply those new methods to rocks from another study area, and is the first detailed geological study of the Newton Range in the New Zealand Southern Alps. The Newton Range is a ~15 km-long, east-west trending range located ~30 km southeast of the town of Hokitika, ~110 km northeast of the Franz Josef-Fox Glacier region, and immediately to the east of the Alpine Fault in the Southern Alps, South Island, New Zealand. The rocks in the Newton Range are mainly derived from Torlesse Terrane accretionary prism greywacke and argillite (Alpine Schist, greyschist), together with a large pods of ultramafic rock (part of the Pounamu Ultramafic Belt (PUB)) and minor associated metabasic layers (greenschist), all metamorphosed to greenschist facies conditions. The dominant mineral assemblage in the greyschist (Qtz + Ms+ Bt ± Chl ± Ep ± Pl ± Ilm ± Ttn ± Grt ± Zrn ± Tur ± Ap ± Cal), much like that found elsewhere in the Southern Alps. As elsewhere in the Southern Alps, the dominant high-grade metamorphic mineral assemblages in the Alpine Schist in the Newton Range are inherited. The mineral assemblages, compositions, and some textures thus record evidence of processes that took place during tectonic events, presumably mainly in Cretaceous time, prior to the formation of the modern Southern Alps, which are forming today by the ongoing oblique continent-continent collision of the Pacific Plate against the Australian Plate at the Alpine Fault. Compositional zoning in garnet from the greyschist is an important record of the metamorphic P-T path traversed by the host rock as the garnet grew. Occasionally, garnet from the study area contains an inmost core (stage 0) of unusual (anomalously high- or low-MnO) composition. The cores with extremely low MnO are possibly detrital in origin, and those with extremely high MnO may perhaps have grown in the early tectonic episode that formed the Otago Schist. Typically, garnet shows the following core- to rim zoning sequence. Stages 1 & 2 show a progressive decrease in MnO and increase in FeO from core to rim, with higher MnO cores present in rocks with higher whole-rock MnO compositions. Stage 3 is characterised by a gradual decrease in CaO and signifies the growth of Ca-bearing oligoclase late in the garnet growth history. Stage 4 is a discontinuous overgrowth characterised by an abrupt increase in CaO. Such overgrowths have in the past been attributed to garnet growth accompanying the development of the Alpine Fault mylonite zone in the late Cenozoic. In the Newton Range they were only observed on garnet adjacent to the main outcrop of the PUB at ~4.5km from the Alpine Fault, far from the mylonite zone, so local element availability during decompression (and possibly fluid flow and/or metasomatism) may have played a part in the growth of these rims. A P-T path for Alpine Schist from the Newton Range has been estimated using detailed garnet composition data measured along core-to-rim transects across individual garnets, together with predicted garnet compositions and P-T pseudosection results calculated using THERMOCALC. The P-T path starts at ~3.5kbar/400°C, where both garnet and albite coexist, and increases in pressure and temperature to ~6.5bar/500°C where garnet coexists with both albite and oligoclase. The estimated peak metamorphic conditions probably correspond to peak metamorphic pressures, unlike in the Franz Josef-Fox Glacier region where peak conditions (~9.2kbar and 620°C) probably coincided with peak metamorphic temperatures.</p>

Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

<p>Metamorphic rocks have the potential to record in their mineral assemblages, mineral compositional zoning, and textures, information about geological changes and processes that occur during tectonic events. Interpretations of metamorphic pressure-temperature (P-T) records have traditionally relied on results of geothermobarometry studies, but that approach is not suitable in every case. Metamorphosed greywacke, which makes up ~95% of the New Zealand Southern Alps, has long proven problematic for traditional geothermobarometry because it develops intractable mineral compositions and/or assemblages, especially at relatively low temperature (greenschist facies) conditions. An alternative forward modelling approach using the computer program THERMOCALC was recently used to extract the first detailed P-T history (P-T path) from such previously intractably difficult "greyschist" rocks from a single site in the New Zealand Southern Alps. The present study is the first attempt to apply those new methods to rocks from another study area, and is the first detailed geological study of the Newton Range in the New Zealand Southern Alps. The Newton Range is a ~15 km-long, east-west trending range located ~30 km southeast of the town of Hokitika, ~110 km northeast of the Franz Josef-Fox Glacier region, and immediately to the east of the Alpine Fault in the Southern Alps, South Island, New Zealand. The rocks in the Newton Range are mainly derived from Torlesse Terrane accretionary prism greywacke and argillite (Alpine Schist, greyschist), together with a large pods of ultramafic rock (part of the Pounamu Ultramafic Belt (PUB)) and minor associated metabasic layers (greenschist), all metamorphosed to greenschist facies conditions. The dominant mineral assemblage in the greyschist (Qtz + Ms+ Bt ± Chl ± Ep ± Pl ± Ilm ± Ttn ± Grt ± Zrn ± Tur ± Ap ± Cal), much like that found elsewhere in the Southern Alps. As elsewhere in the Southern Alps, the dominant high-grade metamorphic mineral assemblages in the Alpine Schist in the Newton Range are inherited. The mineral assemblages, compositions, and some textures thus record evidence of processes that took place during tectonic events, presumably mainly in Cretaceous time, prior to the formation of the modern Southern Alps, which are forming today by the ongoing oblique continent-continent collision of the Pacific Plate against the Australian Plate at the Alpine Fault. Compositional zoning in garnet from the greyschist is an important record of the metamorphic P-T path traversed by the host rock as the garnet grew. Occasionally, garnet from the study area contains an inmost core (stage 0) of unusual (anomalously high- or low-MnO) composition. The cores with extremely low MnO are possibly detrital in origin, and those with extremely high MnO may perhaps have grown in the early tectonic episode that formed the Otago Schist. Typically, garnet shows the following core- to rim zoning sequence. Stages 1 & 2 show a progressive decrease in MnO and increase in FeO from core to rim, with higher MnO cores present in rocks with higher whole-rock MnO compositions. Stage 3 is characterised by a gradual decrease in CaO and signifies the growth of Ca-bearing oligoclase late in the garnet growth history. Stage 4 is a discontinuous overgrowth characterised by an abrupt increase in CaO. Such overgrowths have in the past been attributed to garnet growth accompanying the development of the Alpine Fault mylonite zone in the late Cenozoic. In the Newton Range they were only observed on garnet adjacent to the main outcrop of the PUB at ~4.5km from the Alpine Fault, far from the mylonite zone, so local element availability during decompression (and possibly fluid flow and/or metasomatism) may have played a part in the growth of these rims. A P-T path for Alpine Schist from the Newton Range has been estimated using detailed garnet composition data measured along core-to-rim transects across individual garnets, together with predicted garnet compositions and P-T pseudosection results calculated using THERMOCALC. The P-T path starts at ~3.5kbar/400°C, where both garnet and albite coexist, and increases in pressure and temperature to ~6.5bar/500°C where garnet coexists with both albite and oligoclase. The estimated peak metamorphic conditions probably correspond to peak metamorphic pressures, unlike in the Franz Josef-Fox Glacier region where peak conditions (~9.2kbar and 620°C) probably coincided with peak metamorphic temperatures.</p>

Tectonism and metamorphism along a southern Appalachian transect across the Blue Ridge and Piedmont, USA

ABSTRACT The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time (P-T-t) paths for reconstructing the tectonic history are largely lacking for the southernmost end of the orogen. In this contribution, we describe select field locations in a rough transect across the orogen from Ducktown, Tennessee, to Goldville, Alabama. Metamorphic rocks from nine locations are described and analyzed in order to construct quantitative P-T-t paths, utilizing isochemical phase diagram sections and garnet Sm-Nd ages. P-T-t paths and garnet Sm-Nd ages for migmatitic garnet sillimanite schist document high-grade 460–411 Ma metamorphism extending south from Winding Stair Gap to Standing Indian in the Blue Ridge of North Carolina. In the Alabama Blue Ridge, Wedowee Group rocks were metamorphosed at biotite to staurolite zone, with only local areas of higher-temperature metamorphism. The Wedowee Group is flanked by higher-grade rocks of the Ashland Supergroup and Emuckfaw Group to the northwest and southeast, respectively. Garnet ages between ca. 357 and 319 Ma indicate that garnet growth was Neoacadian to early Alleghanian in the Blue Ridge of Alabama. The P-T-t paths for these rocks are compatible with crustal thickening during garnet growth.

Re-Evaluation of the First Metamorphic P-T Path Using QuiG Barometry and Equilibrium Thermodynamics

Quartz in garnet (“QuiG”) barometry is a relatively new technique that uses physical properties of minerals to estimate the pressure of garnet nucleation and growth history independent of chemical equilibrium. QuiG barometry was used to determine pressures of garnet growth and compared to thermodynamically calculated P-T conditions for two samples (FH-1M and Z3H) from the Lower Shieferhülle (Formation), Tauern Window, Austria. FH-1M was the first sample for which a P-T path was calculated through inversion of chemical zoning in garnet (Selverstone et al., 1984). Mineral Assemblage Diagrams (MADs) and geothermobarometric techniques were used to determine P-T conditions for garnet nucleation and peak metamorphism. No MAD reproduced either the results of Selvserstone et al. (1984) or petrologic observations such as mineral assemblages and likely P-T conditions as determined using independent thermobarometers. Thermobarometrically calculated rim conditions were consistent between our study and previous work in the Lower Schieferhülle. However, without appropriate inclusion assemblages and compositions, the accuracy of calculated core P-T conditions could not be independently assessed using thermobarometry for either rock. QuiG isomekes from both samples are broadly consistent with growth of garnet during exhumation with heating as originally proposed by Selverstone et al. (1984). However, the QuiG isomekes for Z3H suggest that 90% or more of the Z3H garnet grew over small changes in pressure and temperature or along a QuiG isomeke (heating with a slight increase in pressure). These results support the accuracy of prior P-T paths and their tectonic interpretations. However, inconsistencies between QuiG barometry vs. thermodynamic calculations remain unresolved.

Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

Retrograde metamorphic rocks provide key insights into the pressure–temperature (P–T) evolution of exhumed material, and resultant P–T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P–T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P–T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4–1.7 GPa between 500–550 ∘C, retrograde epidote growth at ∼1.3–1.5 GPa between 400–500 ∘C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P–T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P–T-deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes.

The interfacial energy penalty to crystal growth close to equilibrium

Understanding the origin of rock microstructure is critical for refining models of the geodynamics of the Earth. We use the geometry of compositional growth zoning of a population of garnet porphyroblasts in a mica schist to gain quantitative insight into (1) the relative growth rates of individual crystals, (2) the departure from equilibrium during their growth, and (3) the mobility of the porphyroblast-matrix interface. The driving force for garnet growth in the studied sample was exceedingly small and is comparable in magnitude to the interfacial energy associated with the garnet-matrix interface. This resulted in size-dependent garnet growth at macroscopic length scales, with a decrease in radial growth rates for smaller crystals caused by the penalty effect of the interfacial energy. The difference in growth rate between the largest and the smallest crystal is ~45%, and the interface mobility for garnet growth from ~535 °C, 480 MPa to 565 °C, 560 MPa in the phyllosilicate-dominated rock matrix ranged between ~10–19 and 10–20 m4 J–1 s–1. This is the first estimation of interface mobility in natural rock samples. In addition to the complex structural and chemical reorganization associated with the formation of dodecahedral coordination polyhedra in garnet, the presence of abundant graphite may have exerted drag on the garnet-matrix interface, further decreasing its mobility.

Using the elastic properties of zircon-garnet host-inclusion pairs for thermobarometry of the ultrahigh-pressure Dora-Maira whiteschists: problems and perspectives

The ultrahigh-pressure (UHP) whiteschists of the Brossasco-Isasca unit (Dora-Maira Massif, Western Alps) provide a natural laboratory in which to compare results from classical pressure (P)–temperature (T) determinations through thermodynamic modelling with the emerging field of elastic thermobarometry. Phase equilibria and chemical composition of three garnet megablasts coupled with Zr-in-rutile thermometry of inclusions constrain garnet growth within a narrow P–T range at 3–3.5 GPa and 675–720 °C. On the other hand, the zircon-in-garnet host-inclusion system combined with Zr-in-rutile thermometry would suggest inclusion entrapment conditions below 1.5 GPa and 650 °C that are inconsistent with the thermodynamic modelling and the occurrence of coesite as inclusion in the garnet rims. The observed distribution of inclusion pressures cannot be explained by either zircon metamictization, or by the presence of fluids in the inclusions. Comparison of the measured inclusion strains with numerical simulations shows that post-entrapment plastic relaxation of garnet from metamorphic peak conditions down to 0.5 GPa and 600–650 °C, on the retrograde path, best explains the measured inclusion pressures and their disagreement with the results of phase equilibria modelling. This study suggests that the zircon-garnet couple is more reliable at relatively low temperatures (< 600 °C), where entrapment conditions are well preserved but chemical equilibration might be sluggish. On the other hand, thermodynamic modelling appears to be better suited for higher temperatures where rock-scale equilibrium can be achieved more easily but the local plasticity of the host-inclusion system might prevent the preservation of the signal of peak metamorphic conditions in the stress state of inclusions. Currently, we cannot define a precise threshold temperature for resetting of inclusion pressures. However, the application of both chemical and elastic thermobarometry allows a more detailed interpretation of metamorphic P–T paths.