Date of Award

5-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

College/School

College of Science and Mathematics

Department/Program

Earth and Environmental Studies

Thesis Sponsor/Dissertation Chair/Project Chair

Mathew L. Gorring

Committee Member

Stefani Brachfeld

Committee Member

Richard A. Volkert

Abstract

The 1010 Ma Canada Hill Granite represents a small post-orogenic granitoid body that can be found intermingled with Middle Proterozoic rocks in the Hudson Highlands, NY and postdates the thermal peak of the Ottawan Orogeny. The Canada Hill Granite is part of a chemically diverse group of plutonic rocks that formed from crustal melting during late to post-Ottawan dextral transpression and lithospheric thinning.

Thirty-two samples were collected of the Canada Hill Granite and surrounding rocks from the Hudson Highlands, NY in the West Point-Bear Mountain area, from which new major- and trace-element data have been obtained. The Canada Hill Granite occurs in the cores of late folds comprising three small plutons which occur as lenses within migmatitic metapelitic gneisses. The Canada Hill Granite is strongly peraluminous with an aluminum saturation index (A/CNK) average of 1.4 (A/CNK = molar AI2O3 / (CaO + Na2O + K2O). Major element geochemistry identifies the Canada Hill Granite as high in SiO2 (68.4 to 81.7 wt%) with an average of 73.6%, highly variable K2O (0.63 to 9.7 wt%) with an average of 5.4 wt%, low but variable CaO (0.4 to 3.9 wt%) average 1.08 wt%, and a high K2O/Na2O ratio (0.13 to 6.7) with an average 1.8 wt%. Total Fe (reported as Fe2O3) contents are low for most samples (0.16 to 4.6 wt%) and molar FeOt/(FeOt+MgO) ratios are low to moderate (0.6 to 1.0).

The Canada Hill Granite can be broken into three groups; (1) cumulate group, (2) fractionated/evolved melts group, and (3) anatectic granite group based on the presence of a europium anomaly and the corresponding REE depletion/enrichment. The cumulate sample group chondrite-normalized rare-earth-element (REE) patterns are characterized by moderate to steep negative slopes. La/Yb values range from 0.77 to 94 with an average of 31.4. Eu anomalies are generally very highly positive, with Eu/Eu* values of 2.1 to 11.7 and an average of 4.3. This suggests a significant role for feldspar in the petrogenesis of these samples through extensive crystal fractionation and accumulation of a feldspar-rich assemblage (plagioclase + K-feldspar ± quartz). The fractionated/evolved melts group captures the end product of fractional crystallization processes. These rocks formed from a proposed filter pressing method that removed the liquid from the early formed quartz-feldspar cumulate. This group has the highest TiO2 content of any group and in general have high Rb/Sr (1.16 - 1.78), All samples in this group have variable negative Eu anomalies (Eu/Eu* = 0.11 - 0.77). These fractionated melts also tend to be more highly enriched in light rare earth elements (LREE) due to their later timing in the crystallization process. The REE patterns are characterized by steep to very steep negative slopes. La/Yb values range from 4.2 to 452 with an average of 145. A small group of samples represent anatectic granite where variable loss of material was relatively low and represents an initial melt composition before crystallization processes affected the liquid. Samples within this group have small to no europium anomalies (Eu/Eu* = 0.72 - 1.11), and show the least amount of variation in REE concentrations particularly with LREE, and La/Yb slopes are moderate to low (6.3-79) average = 32.9.

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