Archean High-Mg Granodiorite: A Derivative of Light Rare Earth Element-enriched Monzodiorite of Mantle Origin

Abstract

The Roaring River Complex, Superior Province, Canada, contains rocks varying from diorite and monzodiorite to granodiorite which are characterized by high mg-numbers (0.43–0.62), high abundances of Cr (≤150 ppm), Sr (500–2000 ppm), Ba(1000–2500 ppm), and P2O5 (≤0.5 wt.%), low Rb/Sr ratios (001–0.02), and steeply fractionated, subparallel REE patterns (Cen =65–170, Ybn = 3–6) without Eu anomalies. The continuous compositional variation of the rock suite provides a basis for testing the various processes thought to have been important in the extraction of granodiorite magmas from the mantle during the Archean. We consider (1) the relative roles of partial melting, crystallization fractionation, and other processes; (2) the role of garnet or other phases in controlling the steep REE patterns of the rocks; and (3) the chemical and isotopic composition of the source region. The subparallel and decreasing REE patterns with increasing silica, and the ten-fold variation and high abundances of Cr and Ni within the diorite-granodiorite series are not consistent with different extents of melting of basic crust. The scatter in bivariate plots for closely spaced samples does not support simple two-component mixing or liquid immiscibility. The compositional variation can be explained by crystallization differentiation (from 0 to 90%) of monzodioritic magma through separation of dioritic cumulates containing clinopyroxene, hornblende, biotite, plagioclase, K-feldspar, and accessories. The compatibility of the REEs resulted principally from crystallization of sphene and apatite. The parental monzodioritic magmas with their high mg-numbers, Ni, and Cr contents were derived from peridotitic source rocks (mg-numbers>0.80) with low Rb/Sr ratios (<0.02) and light-REE enrichment relative to chondrites. The differences in the REE patterns of monzodiorite samples do not support, nor rule out, garnet in the residue for melting. If the monzodioritic melts were derivatives of other melts, the parent melts would have been similar to high-Mg monzodiorites (‘sanukitoids’) recognized as components of other diorite-granodiorite bodies in the region. An Rb-Sr whole-rock isochron (n = 25) yields a minimum crystallization age of 2623 Ma (±19) with initial 87Sr/86Sr = 070134 (±000004; MSWD=l.8). Sm-Nd isotope data for six rocks yield εNd (2623)= +0.8 ±0.3. The isotope data indicate a source region with long-term Rb/Sr of ∼0.02, similar to depleted mantle, and light-REE depletion relative to chondrites. The peridotite source to the diorite-granodiorite series became light-REE enriched before melting through the addition of a light-REE component of a fluid or melt. In generating Archean granodiorite with suitably high mg-numbers, and Ni, Cr, Sr, Ba, P2O5, and light-REE contents, these data indicate: (1) the importance of crystallization differentiation of high-Mg monzodioritic parent magmas, (2) that the steep REE patterns may be a characteristic of the source rocks, and (3) light-REE-enriched, peridotitic sources were melting and contributing siliceous material directly to the Archean crust.