Petrogenesis Of The Eastern Gabbro, Coldwell Alkaline Complex, Ontario

• Main Author: Shaw, Cliff Sammuel
• Format: Text
• Language: unknown
• Published: Scholarship@Western 1994
• Subjects: Geology; Greenland
• Online Access: https://ir.lib.uwo.ca/digitizedtheses/2473; https://ir.lib.uwo.ca/cgi/viewcontent.cgi?article=3472&context=digitizedtheses

The Eastern Gabbro is the oldest major intrusion in the {dollar}1108\pm1{dollar} Ma Coldwell Alkaline Complex, NW Ontario. The Eastern Gabbro forms a ring dyke surrounding a variety of lithologies ranging from quartz syenite to nepheline syenite. Four subunits make up the Eastern Gabbro, in order of intrusion these are, Gabbronorite, Two Duck Lake Intrusion (TDLI), Layered Gabbro (LG) and the Malpas Lake Intrusion (MLI). The Gabbronorite, TDLI and LG rocks contain plagioclase, olivine, clinopyroxene and minor orthopyroxene. No orthopyroxene was identified in the MLI rocks.;Rock Mg#, and compatible elements, e.g. Ni, decrease in abundance in the sequence Gabbronorite, TDLI, LG, MLI. Incompatible elements, e.g. Zr, Ba and REE, increase in abundance in the same sequence. The rocks of the Gabbronorite are SiO{dollar}\sb2{dollar}-rich and have a different crystallisation history compared to the TDLI and LG rocks. This suggests that the Gabbronorite rocks assimilated silica.;Estimates of the composition of the magma that formed each subunit were made by analyses of fine-grained samples and weighted averaging of cumulate rock compositions. The most primitive magma composition is that of the TDLI, which is similar to a high-Al basalt. This magma underwent fractional crystallisation in an open system with removal, recharge and fractional crystallisation in an expanding or steady-state system. Modelling of this processes using trace elements indicates 3-10 cycles in an expanding magma chamber provides the best fit for fractionation of the TDLI magna to form the LG magma. The MLI rocks could have formed by continued fractionation and are similar in composition to the younger Fe-rich augite syenite.;Internal variations in the composition of the LG rocks are mainly due to variations in the abundance of modal minerals. Mineral chemical variations are a function of crystal accumulation and reaction with interstitial liquid.;The rocks of the Eastern Gabbro are distinct in composition from those of the younger Alkaline Gabbro that occurs in the south central part of the Coldwell Alkaline Complex. The differences suggests that these rocks developed from different magmas.;In the Keweenawan Supergroup, high-Al basalt magmatism is generally considered to have occurred in the period 1097-1085 Ma. The Eastern Gabbro is considerable older than this suggesting that this magma type was present locally thought the Keweenawan.;The association between gabbro and syenite is common in rift zones e.g. Oslo and Greenland. In these cases many of the gabbroic rocks are similar in composition to those of the Eastern Gabbro.