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Abstract: Tectonic architecture of an arc-arc collision zone, Newfoundland Appalachians

High-pressure serpentinization and abiotic methane formation in metaperidotite from the Appalachian subduction, northern Vermont

\n The Appalachian orogen has long been enigmatic because, compared to other parts of the Paleozoic orogens that formed following the subduction of the Iapetus Ocean, direct evidence for ultrahigh-pressure (UHP) metamorphism has never been found. We report the first discovery of coesite in the Appalachian orogen in a metapelite from the mid-Ordovician (Taconic orogeny) Tillotson Peak Complex in Vermont (USA). Relict coesite occurs within a bimineralic SiO2 inclusion in garnet. In situ elastic barometry and trace-element thermometry allow reconstruction of the garnet growth history during prograde metamorphism. The data are interpreted to indicate garnet nucleation and crystallization during blueschist- to eclogite-facies subduction zone metamorphism, followed by garnet rim growth at UHP conditions of > 28 kbar and > 530 ° C. Results provide the first direct evidence that rocks of the Appalachian orogen underwent UHP metamorphism to depths of > 75 km and warrant future studies that constrain the extent of UHP metamorphism.

Evidence for ultrahigh-pressure metamorphism discovered in the Appalachian orogen

Mafic rocks containing sodic-calcic amphibole in the Green Mountain slice (GMS) and Rowe/Prospect Rock slice (R/PRS), Vermont Appalachians were originally subalkaline basalts emplaced as melts during Neoproterozoic rifting of Rodinia that led to the formation of the Iapetus Ocean basin. Relatively high degrees of partial melting of asthenosphere that was highly depleted in incompatible elements produced R/PRS magma(s), which may have been contaminated locally by continental crust and/or fluids. Trace element chemistry suggests that mafic bodies from different locations in the R/PRS may be fractionated magmatic equivalents. Mafic rocks in the GMS formed from magmas produced by relatively low degrees of partial melting of mantle that was relatively enriched in incompatible elements compared to depleted mantle. These enriched melts may have been derived from a plume and/or enriched lithospheric components, potentially continental crust, embedded in depleted upper mantle. A depleted mantle signature preserved locally in the GMS probably reflects increasing asthenospheric input during crustal thinning, opposed to crustal or fluid contamination. Whole-rock minor and trace element data from the GMS and R/PRS are distinct from analyses of glaucophane schist from the Tillotson Peak Complex in northern Vermont, which may be exhumed Iapetan ocean floor. Exhumed mafic rocks in the GMS and R/PRS were formed as lower plate melts prior to relatively high pressure subduction zone metamorphism, providing evidence for subduction of the Laurentian margin, not subduction erosion. Low pressure greenschist facies mafic rocks that occur structurally between the GMS and R/PRS were sourced from depleted or highly depleted mantle in a supra-subduction zone environment, potentially a forearc or backarc basin; an ophiolitic origin is equivocal. Geochemical and isotopic data and interpretations are compatible with rift-related tectonomagmatic models for the peri-Laurentian realm of the northern Appalachians.

Mafic whole-rock geochemistry and neodymium isotopes, Green Mountain and Rowe/Prospect Rock slices, Vermont Appalachians

Subduction zones are tectonically active regions that produce seismicity and volcanism during plate convergence and ultimately recycle crustal material into the mantle. Since these regions dictate many global scale tectonic and geochemical processes (i.e. orogenesis, volatile flux into the mantle, etc.), it is important to understand the depth-temperature conditions of mineral reactions during the subduction-exhumation metamorphic cycle. (Ultra)high-pressure ((U)HP) metamorphic rocks, such as blueschists and eclogites, are formed during metamorphism of subducted crust and sediments along relatively cold geothermal gradients. (U)HP metamorphic rocks may be incorporated into the subduction zone accretionary wedge and exhumed, thereby providing a direct records of subduction zone conditions and processes. This PhD research is focused on the determination of pressure (P) – temperature (T) conditions of exhumed (U)HP subduction zone metamorphic rocks in order to better understand the metamorphic conditions of geologic processes within subduction zones. This dissertation contains three separate studies which combine multiple thermobarometric methods to better constrain the petrologic history of exhumed subduction zone metamorphic rocks. Elastic thermobarometry, trace element thermobarometers, and numerical modeling approaches were integrated to determine P–T(–t) histories of (U)HP metamorphic rocks. In the first chapter of this dissertation, multi-diffusion domain numerical modelling of white mica Ar/Ar and thermobarometric data were used to forward model continuous P–T–t paths of blueschist block exhumation from the western Baja terrane of Mexico. Results from this chapter provide new insights into the application of white mica P–T–t numerical modeling to constrain exhumation histories of (U)HP metamorphic rocks. In the second chapter, strain-based quartz-in-garnet elastic thermobarometric data was combined with Ti concentration measurements and a Ti-in-quartz solubility model to estimate P–T conditions of inclusion entrapment in garnet from a quartzofeldspathic gneiss from the (U)HP terrane of eastern Papua New Guinea. The quartz-in-garnet and Ti-in-quartz (QuiG-TiQ) method gives P–T constraints from a single mineral and does not introduce temperature estimates external of the host-inclusion system. Results from this chapter give new insights into the use of elastic thermobarometry to determine conditions of metamorphic mineral growth and inclusion entrapment. In the third chapter, the first characterization of mineralogical evidence for UHP metamorphism in the Appalachian orogen is presented. Multiple thermobarometric methods, including Zr-in-rutile trace element and quartz-in-garnet elastic thermobarometry, were combined with petrologic observations to characterize the prograde metamorphic conditions of garnet growth during subduction. Applications of Elastic Modeling, Thermobarometry, and Thermal History Modeling to (Ultra)high-Pressure Metamorphic Rocks by Joseph Patrick Gonzalez BSc. Geology, University of Wyoming 2015, BSc. Environment and Natural Resources, University of Wyoming 2015 Dissertation Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Earth Sciences Syracuse University December 2019 Copyright © Joseph Patrick Gonzalez 2019 All Rights Reserved

Applications of Elastic Modeling, Thermobarometry, and Thermal History Modeling to (Ultra)high-Pressure Metamorphic Rocks

A tectonized ultramafic-mafic-pelitic package southwest of the Burgess Branch Fault Zone in the Rowe/Prospect Rock slice in Stockbridge, Vermont preserves Taconian subduction zone metamorphism. The polydeformed package consists of carbonatized and chloritized serpentinite lozenges that are faulted between graphitic phyllite and an intercalated zone of mafic greenstones and pelitic schists that retain primarily Taconian structures. Amphiboles zoned chemically in greenstones display decreases in PL (NaM4) and TK (AlVI + Fe3+ + 2Ti + Cr) from cores to rims. Pseudosection and amphibole isopleth calculations show that different greenstone bodies experienced peak subduction zone metamorphism at different depths between ∼19 and 32 Km (0.55–0.95 GPa) and/or at different temperatures between ∼425 and 500 °C. Barroisite and winchite cores preserve the highest pressure and lowest temperature peak metamorphism, which is interpreted as lower blueschist facies. All greenstones followed decompression cooling paths to lower greenschist facies where actinolite rims grew. Assembly of the ultramafic-mafic-pelitic package probably occurred in a Taconian exhumation channel containing mantle serpentinite, oceanic crust, and clastic rocks of the Laurentian margin and Iapetan Ocean basin.

A tectonized ultramafic-mafic-pelitic package in Stockbridge, Vermont: Metamorphism resulting from subduction and exhumation

Mineralogical and fluid inclusion constraints on the formation of the Karakorum Migmatite: implications for H2O-fluxed melting and exhumation of the South Tibetan Crust

Net-Transfer Reactions and Modal Spaces For Ultramafic Slivers, Vermont Appalachians, USA

Bimodal igneous suites and associated immature clastic sedimentary rocks are characteristic of many orogenic gold-mineralized, crustal-scale fault zones globally. In the central Newfoundland Appalachian orogen, the Rogerson Lake Conglomerate belt and Botwood basin are Late Silurian (Wenlock to Pridoli), fault-controlled sedimentary rock sequences and magmatic suites closely associated with orogenic gold mineralization; however, the spatio-temporal evolution of faulting and associated sedimentation and magmatism are not fully resolved.
U–Pb zircon geochronological results were obtained by using an integrated approach employing LA-ICPMS (laser ablation-inductively coupled plasma mass spectrometry) followed by CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) on the same detrital samples. Using this approach, a maximum depositional age for sedimentary rocks of the Rogerson Lake Conglomerate sequence is 421.9 ± 1.0 Ma (Pridoli), which confirms that they are younger than, and stratigraphically overlie, ca. 422–420 Ma igneous rocks exposed along the central Newfoundland gold belt. Towards the stratigraphic middle of the Botwood basin in north-central Newfoundland, a tuffite layer intercalated with graded siltstone produced a maximum depositional age of 427.9 ± 3.1 Ma (Wenlock; Homerian). The age of emplacement of an autobrecciated, flow-banded rhyolite dome of the Charles Lake volcanic belt along the northwestern Botwood basin is 429.3 ± 0.7 Ma (Wenlock; Homerian). The high-precision CA-ID-TIMS zircon data establish a clear link between Wenlock to Pridoli magmatism and sedimentation throughout central Newfoundland. Furthermore, these geochronological results are consistent with a structural model involving the southeastward (present-day coordinates) advancement of a transient extensional fault system across strike of the Exploits Subzone between ca. 429 and 418 Ma, with propagation along strike to the southwest (Rogerson Lake Conglomerate belt) between ca. 422 and 418 Ma. Extensional faulting may have contributed to basin formation, subsidence, and exhumation of pre-Late Silurian rocks of the Exploits Subzone.
Time-transgressive, extension-related magmatism and clastic sedimentation appear to mark the transition between the Salinic and Acadian orogenic cycles along the central Newfoundland gold belt. Transient Wenlock to Pridoli lithospheric extension may have been important for increasing heat and fluid flow in the crust as a prelude to Devonian crustal thickening, fluid focussing, and orogenic gold mineralization.

U–Pb geochronology of Late Silurian (Wenlock to Pridoli) volcanic and sedimentary rocks, central Newfoundland Appalachians: targeting the timing of transient extension as a prelude to Devonian orogenic gold mineralization

Ian W. Honsberger

Papers

Mafic whole-rock geochemistry and neodymium isotopes, Green Mountain and Rowe/Prospect Rock slices, Vermont Appalachians

A tectonized ultramafic-mafic-pelitic package in Stockbridge, Vermont: Metamorphism resulting from subduction and exhumation

Mineralogical and fluid inclusion constraints on the formation of the Karakorum Migmatite: implications for H2O-fluxed melting and exhumation of the South Tibetan Crust

Net-Transfer Reactions and Modal Spaces For Ultramafic Slivers, Vermont Appalachians, USA

U–Pb geochronology of Late Silurian (Wenlock to Pridoli) volcanic and sedimentary rocks, central Newfoundland Appalachians: targeting the timing of transient extension as a prelude to Devonian orogenic gold mineralization