Nd isotope chemistry of Tertiary igneous rocks from Arran, Scotland: implications for magma evolution and crustal structure

Abstract

The geochemistry of Tertiary igneous rocks from Arran, western Scotland, provides a probe for the structure of the crust in the region of the Highland Boundary Fault (HBF). New Nd isotope data, coupled with other geochemical evidence, point to variable contamination of primitive mantle-derived magmas during magmatic differentiation in the crust. Two different isotopic contamination trends are seen. The northern granite was generated by contamination of basic differentiates by crust resembling exposed Dalradian units. Data for the central granite, and several other minor intrusions from south of the HBF, trend towards the reported isotopic signatures of granulite-facies xenoliths from the Midland Valley. However, quartz porphyry intrusions in the south of the island are compositionally similar to the nothern granite, and were probably intruded southwards across the HBF in dykes. Hence the isotopic signatures of the Tertiary intrusions reflect the different character of the crustal units on either side of the fault.

Magma Ascent along a Major Terrane Boundary: Crustal Contamination and Magma Mixing at the Drumadoon Intrusive Complex, Isle of Arran, Scotland

“…These data allowed us to constrain the magma pathways along the major crustal discontinuity of the Highland Boundary Fault. Previous work by Dickin (1994), based on four samples from the Drumadoon suite, suggested the involvement of Grampian Terrane crust in the formation of the rhyolitic magmas. Kanaris-Sotiriou & Gibb (1985) analysed 44 samples (basaltic andesite and rhyolite) for major and trace elements by X-ray fluorescence (XRF).…”

“…isotopic data for the local crust were used where available (crustal data fields are from this study and Dickin et al, 1981;Dickin & Bowes, 1991;Halliday et al, 1993;Dickin, 1994) and further crustal samples (n ¼ 8; see Table 4, below) were analysed to bridge any gaps in the dataset.…”

“…The igneous rocks are displaced from mantle values towards crustal compositions. Crustal data fields are from this study, Dickin et al (1981), Dickin & Bowes (1991), Halliday et al (1993) and Dickin (1994). isotopic values to sample DD-QFP-19, the rhyolitic end-member.…”

Latest Paleoproterozoic (ca. 1.8–1.6 Ga) extensional tectonic setting in the Dunhuang terrane, NW China: Evidence from geochronological and geochemical investigations on A-type granite and metamafic rock

“…Several petrogenetic models have been proposed for possible magma sources of A-type granites: (1) fractional crystallization of mantlederived basaltic magma (e.g., Turner et al, 1992;Smith et al, 1999;Anderson et al, 2003); 2partial melting of granulite-facies metasedimentary rocks under high temperature (Collins et al, 1982); (3) further differentiation or crustal contamination of syenite magma formed by mixing of mantle-derived alkaline magma and crustal materials (Dickin, 1994;Charoy and Raimbault, 1994); (4) partial melting of continental crust of tonalitic to granodioritic composition (Creaser et al, 1991); (5) partial melting of lower continental crust caused by thermal upwelling of mantle (Anderson and Bender, 1989;Frost and Frost, 1997;Wu et al, 2002); and (6) mixing of mantle-derived mafic and crustal-derived granitic magmas (Yang et al, 2006). Our granite samples have high alkaline but relatively low Al 2 O 3 contents with metaluminous characteristics, excluding the metasedimentary-melting origin.…”

The Geochemical and Zircon Trace Element Characteristics of A-type Granitoids in Boziguoer, Baicheng County, Xinjiang

“…There are generally five views regarding differentiation models: (1) the differentiation of mantlederived alkaline magmas produces A-type residual granitic melts (Loiselle et al, 1979;Eby, 1990Eby, , 1992Pearce et al, 1984); (2) the differentiation of mantle-derived tholeiitic magmas (Frost et al, 1997;Turner et al, 1992;Frost et al, 1999); (3) the formation of alkaline granites because of remelting by rocks derived from the mantle (Zhao et al, 1996); (4) interaction between mantlederived alkaline magmas and crustal materials form magmas with syenite composition, which experience further differentiation or mixing with crustal materials (Dickin, 1994;Charoy et al, 1994;Litvinovsky et al, 2000;Litvinovsky et al, 2002;Zhu et al, 2006); and (5) A-type granites likely evolve by differentiation of transitional alkaline mafic to intermediate magma compositions (Bonin, 2007). The three rocks types of this study have a high degree of consistency with respect to petrological and geochemical characteristics, the…”