Hornblende geothermometry of amphibolite layers of the Popple Hill gneiss, north‐west Adirondack Lowlands, New York, USA

Abstract

The exchange reaction tschermakite+2 diopside+2 quartz=tremolite+2 anorthite (HPCQ), in combination with the thermodynamic database TWQ (version 1.02, Berman, 1991), has been evaluated for its usefulness as a geothermometer. This reaction, which is both water conserving (independent of water fugacity) and which does not require the presence of garnet, is well suited for studying pyroxene‐bearing amphibolites. As an application of this geothermometer, we have re‐examined the amphibolites occurring in the Popple Hill gneiss of the Adirondack Lowlands of New York, USA, to better understand the magnitude of temperature variation preserved in the amphibolites themselves in this classic locality. At an assumed constant pressure of 7 kbar, the temperatures range from 619 to 682 °C from Edwards to Pierrepont and are uncorrelated with either distance along the strike of the region or with modal mineralogical variations. Hornblende exhibits a narrow compositional range suggesting that there has been little or no thermal gradient along the strike of the Lowlands. Temperatures recorded just north of Colton are, however, distinctly higher (694–758 °C). Although it is likely that the Popple Hill gneiss amphibolites experienced some effects of progressive metamorphism, particularly in the vicinity of Colton, the variations in modal mineralogy are most likely the result of such factors as local variations in the bulk chemistry of the protolith and in the fugacity of H2O due to infiltration of diluting species (e.g. CO2, CH4), rather than a regional temperature variation. Temperatures recorded by the HPCQ geothermometer reported here are similar in magnitude and geographic trend to those reported for graphite–calcite carbon‐isotope thermometry by Kitchen & Valley (1995), suggesting that peak metamorphism in the Adirondack Lowlands involved laterally extensive and fairly uniform isotherms.

Lu-Hf and PbSL geochronology of apatites from Proterozoic terranes: A first look at Lu-Hf isotopic closure in metamorphic apatite

“…Liogys and Jenkins (2000), among others, have shown that this area reached metamorphic temperatures as high as 760°C. The PbSL age is therefore not directly comparable to existing ages from the main parts of the Lowlands where maximum temperatures were in the order of 650 to 700°C (section 2.1).…”

“…Peak metamorphism conditions in the Lowlands reached amphibolite facies under low H 2 O activity with pressures close to 7 kbar and temperatures around 650 to 700°C (Kitchen and Valley, 1995;Liogys and Jenkins, 2000). Higher temperatures were reached near the Carthage-Colton zone (760°C; Liogys and Jenkins, 2000), and in the southeastern areas of the Lowlands due to contact metamorphism induced by the 1153 Ma intrusion of the Diana syenite (ϳ900°C; Powers and Bohlen, 1985).…”

“…Higher temperatures were reached near the Carthage-Colton zone (760°C; Liogys and Jenkins, 2000), and in the southeastern areas of the Lowlands due to contact metamorphism induced by the 1153 Ma intrusion of the Diana syenite (ϳ900°C; Powers and Bohlen, 1985).…”

Experimental study of metamorphic reactions and dehydration processes at the blueschist–eclogite transition during warm subduction

“…Background, scale factor, cell dimensions and Lorentzian coefficients ‘LX’ (particle size) were refined. Although the reported uncertainties (precision) for phase abundances in Rietveld refinements are very low (<1 wt%), the observed accuracy in weight percentages of minerals as calculated for mixtures of natural minerals by Liogys and Jenkins (2000) could deviate 3–4 wt% on average from that of the known proportions. The uncertainties for the synthetic mineral mixtures in this study are smaller (1–3 wt% as shown in Table 4) since they usually do not have such high preferred orientations as natural minerals.…”