PETROGRAPHY AND GEOCHEMISTRY OF THE TWIN LAKES GRANODIORITE, CENTRAL SAWATCH RANGE, COLORADO

The Twin Lakes granodiorite is located in the central portion of the Sawatch Range in Colorado, and outcrops over an area of approximately 130 km². It is of Paleocene age and intrudes a package of Pre-Cambrian gneisses, schists and foliated granites. Gravity studies (Case 1964) suggest that the Twin Lakes rocks along with other granitoids of the area may be the exposed top of a larger mass of batholithic proportions. Wilshire (1969) recognized distinct mineral layering in the granodiorite, which resembles similar petrographic features evident in plutons, such as the Mt Waldo and Deer Isle of coastal Maine, where magma mixing and mingling has been a dominant process during their crystallization. This study aims to re-evaluate the field relationships and petrography of the Twin Lakes granodiorite and, along with geochemical data, examine possible modes of origin for the mineral layering.

The rocks of the Twin Lakes pluton are predominantly coarse grained, light brown colored, porphyritic granodiorites with a CI of 10 - 15 and abundant k-feldspar phenocrysts. These range from 1 to 5 cm in size and generally lack a preferred orientation. The groundmass is medium to coarse grained and predominated by plagioclase feldspar, quartz, biotite and minor hornblende and k-feldspar along with accessory amounts of sphene. Other minor petrographic variants are evident within the main body of porphyritic granodiorite and these are more leucocratic granites and lack the large phenocrysts of the main phase. The main areas of mineral layering or schlieren are exposed in the north-central and eastern parts of the intrusion adjacent to contacts with the country rocks. They range in width from veins < 5cm to layers over 1m wide, and have variable dispositions. Mineralogically they vary from being feldspar rich, containing both plagioclase and orthoclase, to more melanocratic layers with higher abundance of biotite and hornblende along with sphene.

The development of mineral layering or schlieren is often attributed to the separation of crystals from liquid during convective flow in a dynamic magma chamber. However, the preponderance of the layering in the Twin lakes intrusion close to its contact may suggest that upward flow and phase separation was restricted to the marginal zones of the pluton.