Isotope, trace element and major element geochemistry of Tertiary igneous rocks, Isle of Arran, Scotland

Abstract

Pb- and Sr-isotope, major element and trace element analyses are presented for 38 Tertiary igneous rocks and older country rocks from Arran. Tertiary igneous rocks define silica-undersaturated and -oversaturated differentiation trends, but acidic and basic examples of the latter trend, exemplified in composite dykes, are separated by a wide Daly gap. In contrast, Pb-, and to some extent Sr-isotope compositions of these rocks display continuous variation, almost independent of major element composition.Isotopic variations are interpreted as resulting principally from crustal contamination of the differentiation products of mantle-derived basic magma. A dominant rôle by mantle heterogeneity or crustal anatexis in producing the isotopic variation is ruled out. Contamination is believed to have occurred primarily after differentiation, but some rocks, such as the fine unit of the Northern Granite, display evidence of post-contamination differentiation.Tertiary igneous rocks define an array on a Pb/Pb isochron diagram whose slope corresponds to an apparent age of 770±180 Ma (2σ). This points to the existence of a late Proterozoic basement complex under Arran.Acidic and basic members of composite dykes are interpreted as basaltic differentiates which have been variably affected by contamination, yielding wide ranges of isotopic composition. Composite intrusion is ascribed to the simultaneous presence of acidic and basic magma in the crust.

Strontium isotope geochemistry of Tertiary igneous rocks, NE Ireland

“…If the acid magmas were somehow derived from the basic magmas, then the Sr isotope data indicate that the acid magmas were selectively contaminated with crustally derived STSr. Inverse hyperbolic mixing trends, such as those depicted in Figure 1 for the Slieve Gullion centre, support the selective contamination hypothesis and are clearly apparent in Sr isotope data reported for Mull (Walsh et al 1979), Arran (Dickin et al 1981) and Skye (Moorbath & Thompson 1980). At the Slieve Gullion centre there is a definite temporal trend in initial S~Sr/S6Sr ratios of the acid rocks to be observed.…”

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

“…Samples were selected for analysis from the suite studied by Dickin, Moorbath & Welke (1981). These represent the two granitic plutons, composite dykes and sills, and a few other minor intrusions.…”

“…A geochemical, strontium and lead isotope study of the intrusive rocks was reported by Dickin, Moorbath & Welke (1981) but, in this paper, additional constraints on their petrogenesis and on the character of the crust are derived from rare earth element (REE) and Nd isotope data.…”

Revised 40 AR– 39 AR age for granites of the Mourne Mountains, Ireland

“…This age of about 54 Ma is significantly younger than the majority of reliable dates for most parts of the BTIP outside Ireland, which mostly fall in the range 62 to 58 Ma Dickin et al 1981;Mussett 1986;Dickin 1981), as do unpublished dates for other parts of Ireland ( Thompson 1986). These rocks are predominantly basic.…”

“…These rocks are predominantly basic. However, reliable younger dates have been obtained for several widely separated bodies: Eastern Red Hills, of Skye, 53-5 ± 0-4 Ma (Dickin 1981), Sgurr of Eigg, 52-1 ±0-5 Ma (Dickin and Jones 1983), the Conachair Granite of St Kilda, 55 ± 1 Ma (Brooks 1984), and the Lundy Granite 54-8 ± 1-4 Ma (Fitch et al 1969). These, like granites of the Mourne Mountains, are acid rocks, though basic rocks were not entirely absent at these times since most of these bodies are cut by basic dykes.…”