Peculiarities in the behavior of trace and rare earth elements during granitization and migmatization of the Elgakan complex (East Siberia, Nyukzha River)

Abstract

The behavior of major elements in the process of Neoarchean amphibolite-facies metamorphism was considered previously for occasionally pyroxene-bearing biotite‐amphibole‐plagioclase schists and amphibolites, which occur in the middle and lower reaches of the Nyukzha River and are identified as the Elgakan Unit [1]. Ultrametamorphism took place in several stages. At the first stage, plagiogranite gneisses (Lc 1 ) and rocks of an intermediate composition (sch*) were sequentially developed from the volume replacement of schists (sch). At the second stage, the stromatic, network, and coarse-banded migmatite leucosomes (Lc 2 , Lc 3 , and Lc 4 , respectively) developed progressively in schists and plagiogranite gneisses [1, 2]. The two stages were separated by emplacement of intermediate and silicic dikes and minor intrusions. This paper is focused on behavior of trace and rare earth elements in these processes. Without dwelling on the major element geochemistry of crystalline schists and amphibolites (27 analyses), let us note that their compositions correspond to basalt‐trachybasalt and basaltic andesite‐basaltic trachyandesite in the TAS diagram: SiO 2 50.10 ± 3.50 and 55.40 ± 1.85 wt %, respectively; (Na 2 O + K 2 O) 5.22 ± 1.86 and 5.48 ± 1.27 wt %, respectively. Correlation links are typical of igneous rocks of this composition: Si displays negative correlation with Fe 3+ , Fe 2+ , Mn, Mg, and Ca; positive correlation with Na; and the absence of significant correlation with K, Ti, and Al. The absence of significant positive correlation with Ca, Na, and Al testifies to the insignificant effect of plagioclase fractionation. The Lc 1 group (26 samples) includes plagiogranite and granite gneisses with biotite (occasionally, amphibole). In terms of composition, this group varies from tonalite to trondhjemite and granite with predominance of rocks containing 68‐73 wt % SiO 2 . In terms of proportions of normative Ab, An, and Or, the distribution of samples is as follows: trondhjemite 39%, granite 23%, and tonalite and granodiorite 19% each. The sum total of alkali metals shows a positive correlation with K and a negative correlation with Na, resulting in a negative correlation between K and Na (‐0.60). In terms of ASI values, most samples fall within a range of 0.9‐1.2 (maximum 1.0‐1.1). Four samples (out of 26) are characterized by ASI = 1.2‐1.4. The Fe index ( F ) varies from 50 to 70%, and the degree of Fe oxidation ( f 0 ) is 10‐40% (maximum 20‐30%). Si has negative correlation with most of the major elements but no correlation with Na and K. Leucosomes Lc 2 (20 samples) always contain >68 wt % SiO 2 . In comparison with Lc 1 , they are characterized by the prevalence of trondhjemite (50%), a higher content of granite (35%), and a lower content of tonalite (15%). The correlation between K and Na is negative (‐0.58). Samples with ASI = 1.0‐1.1 and 1.2‐ 1.3 are most abundant. In terms of F and f 0 , these rocks are close to Lc 1 but marked by more mafic compositions and higher percentages of rocks with elevated contents of alkali metals. Si is negatively correlated with Al, Fe 2+ , and Mg. Rocks of group Lc 3 (8 samples) are similar to Lc 1 in composition. They differ from Lc 2 in their higher Mg and lower Si contents. Group Lc 4 (14 samples) is close in many parameters to Lc 2 . However, the percentage of granite is higher (64%), while tonalite (7%) and trondhjemite (29%) are subordinate. K and Na are not correlated. As in the preceding group, Si lacks any significant correlation with all major elements except Al. The samples are uniformly distributed in three ASI intervals (1.0‐1.1, 1.1‐1.2, and 1.2‐1.3). The maximums of F (60‐70%) and f 0 (20‐30%) are distinct. The compositional variation of basic rocks during the granitization and evolution of leucosomes may be traced by comparing the average major element contents in the rock series from the basic protolith to the granitized basic rocks, plagiogranite gneisses, and leu