Geochemical constraints on the genetic relationship between A-type peralkaline granite
and anorthosite from the Neoarchean Keivy alkaline province, NE Baltic Shield
Zozulya D.R.*, Eby G.N.**
* Geological Institute, Kola Science Centre, Apatity, Russia; ** University of Massachusetts Lowell, USA.
The genetic relationship of anorthosites and of A-type granites and intermediate alkaline rocks is debated for well known localities from Gardar province (Greenland), Air ring complex (N. Niger), Younger Granite province (Nigeria), Pikes Peak batholith (Colorado) (Barker et al., 1975; Bonin, 1996; Demaiffe & Moreu, 1996; Upton, 1974, 1996).
A suite of massif-type anorthosites and peralkaline granites is found in the Archean Keivy terrane of the NE Baltic shield. The Keivy terrane is mainly composed of mafic (minor) - intermediate - felsic (most abundant) metavolcanic and metasedimentary rocks that overlie the oldest tonalite-trondhjemite-granodiorite (TTG) basement of the Central Kola Block. According to the geochronological data, the oldest basement of the Central Kola Block was formed during several stages of the Neoarchean endogenic activity and has an isotopic age of 3.0-2.7 Ga. The felsic metavolcanics of the Keivy terrane, which completed the extrusive succession, represent orogenic rocks with a U-Pb zircon age of 2870 Ma, forming from geochemical constraints in island-arc environment. Interformational sheet-like intrusive bodies of peralkaline granites, and spatially confined to them gabbro-anorthosite lopolithes are present in the margins of the Keivy terrane.
The Keivy alkaline province consists of 2650-2660 Ma aegirine-arfvedsonite granites (six massifs of a few hundred meters thicknesses and with the total exposure area of ca. 2500 km2; the largest ones are the West Keivy, Ponoj, White Tundra), 2670 Ma aegirine-augite-lepidomelane-ferrohastingsite syenogranites in margins of some massifs, and 2680 Ma lepidomelane-ferrohastingsite syenite dykes cutting the TTG basement (Zozulya et al., 2005). Small dike-like bodies of 2610 Ma nepheline syenite cut the West Keivy peralkaline granite massif.
The Keivy anorthosite complex consists of several (the largest Tsaga massif is up to the 170 km2) lopolithes and fault-type intrusions composed mainly of anorthosite and gabbro-anorthosite and marginal gabbro-norite and titanomagnetite-rich troctolite bodies. The formation age for them range from 2670 Ma to 2660 Ma (Zozulya et al., 2005).
The Keivy anorthosites have low REE abundances (Ce 5-23 and Yb 1.5-6.8 times chondrites), fractionated REE distributions (chondrite-normalized La/Yb ratios are 4-10) and positive Eu anomalies (Fig. 1). The comagmatic gabbro-norites have similar REE patterns, but no or negligible positive Eu anomalies. As the chondrite-normalized La/Yb ratios do not correlate with REE abundances, an enriched source for the primary magmas is proposed. The Keivy peralkaline granites are extremely enriched in REE (100-1000 times chondrites), show negative Eu anomalies, have normalized La/Yb ratios of 1.5-13 (Fig. 1).
Fig. 1. Chondrite normalized REE distribution patterns for the Keivy gabbro-anorthosite and peralkaline granite.
The granites of the Keivy alkaline province are extremely enriched in Zr (300-1900 ppm), Y (40-150 ppm), Nb (20-150 ppm) and Rb (160-900 ppm), have associated Zr-REE ore occurrences, are very low in Sc (0.3-1.3 ppm) and Sr (10-30 ppm), and high Ga/Al ratios. On standard trace element discrimination diagrams (Whalen et al., 1987; Pearce et al., 1984; Eby, 1990) the Keivy peralkaline granites plot as within-plate or post-collisional A-type granitoids, and nepheline syenites fall to OIB field. The least evolved syenogranites plot in the EM2-field on the εSr - εNd diagram.
The anorthosites show high compatible element (Sc, 25-40 ppm and Sr, 460-670 ppm) abundances and very low incompatible elements (Zr, 60-100 ppm, Y, 4-15 ppm, Nb, 8-12 ppm, Rb, 15-30 ppm), but the same Y/Nb, Ce/Nb, Yb/Ta ratios as the granites (Fig. 2). The enriched source for the Keivy anorthosites has low εNd (-0.15 to -0.24) and low Y/Nb (0.6-1.3) and Ce/Nb (1-3) ratios. The OIB-like source for anorthosites is enhanced also by the similar Nb/Zr and Ba/Zr ratios (Fig. 2).
The close temporal and spatial association of the gabbro-anorthosites and the peralkaline granites and their similar magma sources suggest a genetic relationship. One possible model is protracted fractional crystallization of a primary subalkaline/alkaline basalt magma with removal of plagioclase during the early stages of crystallization (forming a Ca- and Al-enriched cumulate, anorthosite) and alkali, iron and HFSE enrichment of the residual melt leading to the peralkaline granites (Fig. 3). In cases, this fractionation has been accompanied by simultaneous crustal contamination. “Plagioclase effect” was favored by anorogenic tectonic environment.
Fig. 2. Trace element discrimination diagrams for Keivy anorthosites (peralkaline granite field shown for comparison).
Fig. 3. Schematic illustration of mineral fractionations and inferred sequence of crystallization
for the Keivy anorthosite – peralkaline granite – nepheline syenite anorogenic suite.
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