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Abstract We investigated the interplay between deformation and pluton emplacement with the goal of providing insights into the role of transpression and arc magmatism in forming and modifying continental arc crust. We present 39 new laser-ablation–split-stream–inductively coupled plasma–mass spectrometry (LASS-ICP-MS) and secondary ion mass spectrometry (SIMS) 206Pb/238U zircon and titanite dates, together with titanite geochemistry and temperatures from the lower and middle crust of the Mesozoic Median Batholith, New Zealand, to (1) constrain the timing of Cretaceous arc magmatism in the Separation Point Suite, (2) document the timing of titanite growth in low- and high-strain deformational fabrics, and (3) link spatial and temporal patterns of lithospheric-scale transpressional shear zone development to the Cretaceous arc flare-up event. Our zircon results reveal that Separation Point Suite plutonism lasted from ca. 129 Ma to ca. 110 Ma in the middle crust of eastern and central Fiordland. Deformation during this time was focused into a 20-km-wide, arc-parallel zone of deformation that includes previously unreported segments of a complex shear zone that we term the Grebe shear zone. Early deformation in the Grebe shear zone involved development of low-strain fabrics with shallowly plunging mineral stretching lineations from ca. 129 to 125 Ma. Titanites in these rocks are euhedral, are generally aligned with weak subsolidus fabrics, and give rock-average temperatures ranging from 675 °C to 700 °C. We interpret them as relict magmatic titanites that grew prior to low-strain fabric development. In contrast, deformation from ca. 125 to 116 Ma involved movement along subvertical, mylonitic shear zones with moderately to steeply plunging mineral stretching lineations. Titanites in these shear zones are anhedral grains/aggregates that are aligned within mylonitic fabrics and have rock-average temperatures ranging from ∼610 °C to 700 °C. These titanites are most consistent with (re)crystallization in response to deformation and/or metamorphic reactions during amphibolite-facies metamorphism. At the orogen scale, spatial and temporal patterns indicate that the Separation Point Suite flare-up commenced during low-strain deformation in the middle crust (ca. 129–125 Ma) and peaked during high-strain, transpressional deformation (ca. 125–116 Ma), during which time the magmatic arc axis widened to 70 km or more. We suggest that transpressional deformation during the arc flare-up event was an important process in linking melt storage regions and controlling the distribution and geometry of plutons at mid-crustal levels.
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WHEN DOES A GRANITE BECOME GRANITIC? A RECORD OF MAJOR AND ACCESSORY MINERAL RE-EQUILIBRATION FROM SUPER- TO SUB-SOLIDUS WITHIN THE HALF DOME GRANODIORITE, SIERRA NEVADA, CA
The Tuolumne Intrusive Suite (TIS), Sierra Nevada, California, accumulated magmatic rock from 95 to 85 Ma. Ar-Ar biotite dates require that temperatures within the TIS remained above ~300°C until ~79 million years ago. The protracted thermal history resulted in five texturally and chemically distinct units that young towards the center and was recorded by chemical and isotopic re-equilibration of the minerals. Challener and Glazner (2017) demonstrated that amphibole phenocrysts from the Half Dome Granodiorite (Khd) experienced greenschist-facies metamorphism. Amphibole phenocrysts host abundant inclusions of biotite, chlorite, feldspar, titanite, epidote, and apatite, which are interpreted to have crystallized via breakdown of magnesiohornblende. Additionally, Al zoning suggests fracturing and subsequent healing of the amphibole crystals occurred at near- or subsolidus temperatures. New EPMA and LASS-ICP-MS analyses of texturally related amphibole, titanite, feldspar, and biotite from the equigranular Khd place limits on the timing of amphibole breakdown and contextualize the low-temperature re-equilibration of many of the major minerals in the rock. Most of the amphiboles analyzed contain 0.5–6 wt. % Al2O3 corresponding to actinolite compositions, while feldspar pairs record ~475 ºC apparent temperatures. Titanite grains (re)crystallized between 91–80 Ma and contain 25–825 ppm Zr, which correspond to apparent temperatures between 550–710 ºC (150 ± 50 MPa, aTiO2 = 0.5 ± 0.1). The distribution of Zr in titanites is bimodal with the majority having <200 ppm Zr. Titanites younger than 87 Ma have decreasing Zr content and titanites included within actinolite amphibole contain the lowest Zr content (25–50 ppm) and youngest dates (85–80 Ma). Melt-present crystallization of titanite began at ~91–90 Ma, followed by both near and subsolidus (re)crystallization from ~88–86, concluding with titanite growth via hornblende breakdown from 82–80 Ma. These data taken together with previous investigations provide a continuous record of the rock’s chemical evolution driven by incremental emplacement and subsequent episodic autometamorphism of the equigranular Khd, and critically, any inferences regarding magmatic processes in the TIS must first account for the metamorphic re-equilibration of the rock.
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- Award ID(s):
- 1853496
- PAR ID:
- 10518033
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Journal Name:
- Geological Society of America Abstracts with Programs
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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