PETROLOGY, GEOCHEMISTRY, GEOCHRONOLOGY AND TECTONIC HISTORY OF NICOLA HORST, B.C., CANADA

The Nicola Horst (NH), in the southern part of Intermontane Belt of British Columbia, includes two types of units: stratified rocks and intrusive rocks. The stratified rocks include (a) Metaconglomerates accompanied by black graphitic mica schist, (b) Meta-Nicola rocks consisting of amphibolite and biotite-amphibole schists and (c) Intermediate metavolcanic rocks, mainly metavolcaniclastic (probably ignimbrites) and intrusive porphyries.The bulk of the NH is intrusive rocks, which are medium to coarse-grained plutonic rocks, predominantly granodiorite but ranging from biotite granite, hornblende biotite tonalite to gabbro. These rocks are of three distinct ages: Late Triassic, Jurassic and Paleocene. From thin sections and hand specimen, there is textural and mineral evidence of one peak amphibolite grade syn-kinetic metamorphism throughout the horst, which probably took place during Paleocene, before the emplacement of Paleocene granodiorites, which were not affected by it. From geothermobarometry, the peak metamorphic temperatures and pressures, indicating amphibolite grade, are 475- 525°C and 3.1-3.6 kbars. Detailed geochemistry shows the Triassic metavolcanics to be from a calc-alkaline dominated volcanic arc type of tectonic setting. The Triassic, Late Jurassic – Early Cretaceous and Paleocene intrusives show the same volcanic arc type of setting. There is not a continuum in the magmatic history but each magmatism was a separate event. Earlier it had been concluded that the tectonic setting of Late Triassic and Early Jurassic volcanism in Quesnellia had been subduction related and this can probably be now extended to the Paleocene. An exhumation history of the NH can be envisioned by using the Ar/Ar biotite geochronology data. Assuming that the 300°C (biotite closure temperature) isotherm before exhumation was horizontal, the data indicates that it took 50.3 Ma years for the northern and western part of the NH to be exhumed to the surface from the time it passed through the biotite closure temperatures. The eastern part took 56.5 Ma to get to surface and the southern part of the NH took 63.8 Ma passing through the biotite closure temperatures to 25°C, thus indicating that the exhumation of the NH was not uniform but differential.The average exhumation age calculated from the data is 0.38 km/Ma.