A petrographic study of contact facies of granitic rocks with limestone

A petrographic study is made of granitic rocks which are closely associated with crystalline limestone in north-west British Columbia and south-west Yukon. A description of Contact facies between the two rock types is emphasized by the author with the aim of showing endomorphic and exomorphic effects. It was found that the chief endomorphic effect on the granitic rock was the development of pyroxene as the mafic mineral in place of the usual biotite and hornblende. No change in the composition of the plagioclase feldspar was noted except in specimens taken in close proximity to limestone contacts. In these specimens the plagioclase was found to be abnormally calcic. Considerable quartz, orthoclase, actinolite, and clinozoisite also mark the contact facies of the granitic rock. Recrystallization, and the development of the common skarn minerals, diopside, garnet, and wollastonite have taken place in the limestone. The skarn minerals are restricted to a narrow zone adjacent to granitic rock and along joints in the limestone. A number of hypotheses are given with evidence for and against in an attempt to explain the genesis of the various rock types. Assimilation of the schists and gneisses but not limestone is believed by the author to be the most likely hypothesis to explain the presence of the limestone inclusions in granitic rock. It is postulated that the pyroxene developed as a replacement or metasomatic mineral after the initial stages of the intrusion by the granitic magma. In conclusion the introduction of quartz and orthoclase and the development of actinolite and clinozoisite are thought to be related to a late stage of the igneous activity.