Contact metamorphic P–T–t paths from Sm–Nd garnet ages, phase equilibria modelling and thermobarometry: Garnet Ledge, south‐eastern Alaska, USA

Abstract

Sm–Nd garnet‐whole rock geochronology, phase equilibria, and thermobarometry results from Garnet Ledge, south‐eastern Alaska, provide the first precisely constrained P–T–t path for garnet zone contact metamorphism. Garnet cores from two crystals and associated whole rocks yield a four point isochron age for initial garnet growth of 89.9 ± 3.6 Ma. Garnet rims and matrix minerals from the same samples yield a five point isochron age for final garnet growth of 89 ± 1 Ma. Six size fractions of zircon from the adjacent pluton yield a concordant U–Pb age of 91.6 ± 0.5 Ma. The garnet core and rim, and zircon ages are compatible with single‐stage garnet growth during and/or after pluton emplacement. All garnet core–whole rock and garnet rim‐matrix data from the two samples constrain garnet growth duration to ≤5.5 my. A garnet mid‐point and the associated matrix from one of the two garnet crystals yield an age of 90.0 ± 1.0 Ma. This mid‐point result is logically younger than the 90.7 ± 5.6 Ma core–whole rock age and older than the 88.4 ± 2.5 Ma rim‐matrix age for this sample. A MnNaCaKFMASH phase diagram (P–T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 ± 20 °C and 5 ± 1 kbar. This exceeds the temperature of the garnet‐in reaction by c. 50 °C and is compatible with overstepping of the garnet growth reaction during contact metamorphism. Intersection of three reactions involving garnet‐biotite‐sillimanite‐plagioclase‐quartz calculated by THERMOCALC in average P–T mode, and exchange thermobarometry were used to estimate peak metamorphic conditions of 678 ± 58 °C at 6.1 ± 0.9 kbar and 685 ± 50 °C at 6.3 ± 1 kbar, respectively. Integration of pressure, temperature, and age estimates yields a pressure‐temperature‐time path compatible with near isobaric garnet growth over an interval of c. 70 °C and c. 2.3 my.

Late Cretaceous–Paleocene metamorphic evolution of the Sierra Nevada de Santa Marta: Implications for Caribbean geodynamic evolution

“…The PeT paths integrate rim thermobarometry and pseudosections following the techniques presented by Vance and Mahar (1998), Stowell et al (2001), and Stowell and Tinkham (2003). Garnet rim thermobarometry was used to estimate PeT at peak metamorphic conditions with the average PeT routine of THER-MOCALC (Powell and Holland, 1988;Powell et al, 1998) using externally calculated activities.…”

The use of garnet porphyroblasts to resolve the metamorphic pressure-temperature-deformation (P-T-d) path: An example from the Imjingang belts, South Korea

“…MnO, CaO and Na 2 O were included in the pseudosection calculations for compositions of garnet and plagioclase, therefore the MnNCKFMASH system should be suitable for modeling the P−T history. We used the intersection relationships among isopleths of X Mn , X Fe and X Ca in garnet compositions (e.g., Vance and Holland 1993;Vance and Mahar, 1998;Stowell et al, 2001;Kim and Bell, 2005) via the program THER-MOCALC (Powell and Holland, 1988;Powell et al, 1998) to examine the P−T path revealed by garnet cores that have grown at different stages of development relative to the succession of FIAs. Standard deviation ranges are 712°C in temperature and 0.2−0.5 kbar in pressure.…”

Geochronology: linking the isotopic record with petrology and textures — an introduction

“…In many cases, and particularly for metapelitic rocks, either or both of these approaches is adequate (e.g. Vance & O'Nions 1990;Burton & O'Nions 1991Vance et al 1998b;Vance & Harris 1999;Stowell et al 2001;Th6ni 2002;Stowell & Tinkham 2003). However, it has been known for some time (Jagoutz 1988;Th6ni & Jagoutz 1992;Luais et al 2001;Th6ni 2002) that in (mainly coarse-grained) high pressure metabasic rocks this is often not the case.…”

“…Mezger et al 1989;Christensen et al 1989Christensen et al , 1994Vance & O'Nions 1990Burton & O'Nions 1991Vance et al 1998b;Vance & Harris 1999;Stowell et al 2001;Th6ni 2002;Stowell & Tinkham 2003) but it is not robust. It is the experience of the lead author on this paper that the success rate, as measured by the ability to obtain age precisions of 1-3 Ma, is less than 50%.…”

“…In a case study, Stowell & Tinkham apply this combined approach to polyphase contact and regional rocks from the western Cordillera of North America. The authors present new isotopic and petrological data and integrate it with existing data (Stowell et al 2001) to investigate a terrane with a polyphase history involving plutonism and thrusting related to terrane accretion at 100-60 Ma and three or four contact and regional events. The paper is notable in documenting textures involving the aluminosilicates that are complex but which also provide an abundance of information that is very useful in interpreting the P-T history on pseudosections.…”