Thermochronologic constraints on the tectonic evolution of active metamorphic core complexes, D'entrecasteaux Islands, Papua New Guinea

Abstract

Metamorphic core complexes in the D'Entrecasteaux Islands, Papua New Guinea, formed as the result of active extension at the western end of the propagating Woodlark Basin spreading center. Domes of high-grade metamorphic rocks (i.e., amphibolites, eclogites, and migmatites), intruded by large granodiorite bodies, comprise the lower plate of the D'Entrecasteaux metamorphic core complexes. The domes are transected by kilometer-scale shear zones. A thermochronologic study of the D'Entrecasteaux Islands utilizing K/Ar, 40Ar/39Ar, and fission track techniques has documented the unroofing history of these active metamorphic core complexes. Gneisses in the cores of the domes cooled rapidly (>100øC/m.y.) as indicated by hornblende and biotite 40Ar/39Ar apparent ages of-2.7 to 3.0 Ma and -1.6 to 1.7 Ma, respectively, and apatite fission track ages of-0.4 to 0.9 Ma. 40Ar/39Ar apparent ages on white mica, biotite, and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling (in some cases >500øC/m.y.) and suggest shear zones were active from 4.0 to 3.5 Ma and 1.9 to 1.4 Ma. In general, 40Ar/39Ar mineral ages for retrogressed core zone gneisses, shear zone gneisses, and granodiorites are 2.0 to 3.0 Ma (amphiboles), 1.5 to 1.7 Ma (muscovites), and 1.4 to 1.8 Ma (biotites) and 1.0 to 2.0 Ma (K-feldspars). Apatite fission track ages from core zone gneisses, shear zone gneisses and granodiorites range from 0.4 to 1.0 Ma. Thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes. The granodiorite bodies associated with the D•Entrecasteaux Islands domes represent syn-kinematically emplaced granitoids intruded into

Atmospheric Ar and Ne returned from mantle depths to the Earth’s surface by forearc recycling

“…The eclogite boudins have retrograde amphibolite rinds. Pegmatite veins (3.52 ± 0.10 Ma) intrude the quartzofeldspathic host and are locally concordant with the gneissic foliation (27). Apatite fission track ages for the coesite eclogite host gneiss (0.6 ± 0.2 Ma) indicate exhumation to shallow crustal levels by the middle Pleistocene (27).…”

“…Pegmatite veins (3.52 ± 0.10 Ma) intrude the quartzofeldspathic host and are locally concordant with the gneissic foliation (27). Apatite fission track ages for the coesite eclogite host gneiss (0.6 ± 0.2 Ma) indicate exhumation to shallow crustal levels by the middle Pleistocene (27). Geologic and structural data (25), electron backscatter diffraction measurements, and phase relations (28) provide additional outcrop and regional-scale geochemical (29) and P-T-t-D constraints for the UHP terrane (30).…”

“…The highest temperature step (1,600°C) yielded an 40 Ar/ 39 Ar apparent age of 20.64 ± 0.90 Ma (1σ) with a corresponding high 37 Ar/ 39 Ar ratio, and it likely reflects outgassing of retentive high Ca/K inclusion(s) within phengite. We interpret the youngest 40 Ar/ 39 Ar apparent ages to result from minor neocrystallization on phengite grain boundaries during exhumation, which occurred after crystallization of pegmatitic muscovite at the same outcrop (27). The 40 Ar/ 39 Ar weighted mean age for phengite based on ∼84% of 39 Ar released is interpreted to date the time of phengite crystallization.…”

“…Timing of peak metamorphism based on 40 Ar/ 39 Ar phengite (this study), Lu-Hf garnet (27), and U-Pb zircon (31) ages. Apatite fission track ages (0.6 Ma) from host gneiss at the coesite locality constrain the timing of exhumation to shallow crustal levels (27). Abbreviations for metamorphic facies are blueschist (BS), greenschist (GS), amphibolite (AM), eclogite (EC), and granulite (GR); mineral abbreviations are albite (Ab), jadeite (Jd), and quartz (Qtz).…”

“…One objective was to assess whether results could be interpreted within a geologic and tectonic context or yielded anomalously old 40 (22). Timing of peak metamorphism based on 40 Ar/ 39 Ar phengite (this study), Lu-Hf garnet (27), and U-Pb zircon (31) ages. Apatite fission track ages (0.6 Ma) from host gneiss at the coesite locality constrain the timing of exhumation to shallow crustal levels (27).…”