The origin of the Kennethmont granite-diorite series, Insch, Aberdeenshire

Abstract

SUMMARY. The Insch granite~liorite complex of Aberdeenshire consists of acid and intermediate igneous rocks that are quite distinct from, and younger than, the differentiates of the Insch basic mass.The main components of the complex include two types of uniform granite (one pink, one grey), and a variety of inhomogeneous dioritic rocks (diorite xenoliths in grey granite matrix, granodiorite with residual mafic inclusions, and more uniform diorite). Full chemical analyses for major and trace elements are presented for sixteen rocks representative of all types, together with initial aTSr/SeSr ratios and rare-earth-element distribution patterns for a selection of samples. The pink granite is not chemically related to the remaining members of the complex. Comparison with chemical data for a range of basic igneous rocks of the Insch and Boganclogh basic masses discredits the hypothesis that the diorite rocks were produced by assimilation of any of these in the grey granite magma. A primary diorite magma is recognized as essential to the petrogenesis of the complex.This study is concerned with a group of acid and intermediate igneous rocks that outcrop over an area of several square miles, mostly in the parish of Kennethmont, five miles west of Insch, Aberdeenshire. The area, which is rather poorly exposed, has previously been studied by Read (t923), Sadashivaiah (1954), and Read and Haq (1965). They have described rock types ranging from homogeneous diorites, through xenolithic rocks and inhomogeneous rocks with a relatively acid matrix, to more uniform granites, locally developing a syenitic facies. These are collectively referred to as the Insch granite-diorite complex and are distinct from the differentiated gabbrosyenite sequence of the Insch basic mass which belongs to the 'Younger' Caledonian igneous suite of north-east Scotland. The broad distribution of the complex and adjacent parts of the Insch basic mass are shown by Read et al. (196I), and no major differences were found on remapping by Busrewil (197o). Sadashivaiah (1954) and Read and Haq (I965) discredited on chemical grounds earlier assumptions that the granitic rocks of the complex could be late fractionation products of the Insch basic mass and suggested that they represented a distinctly younger episode of igneous activity. They further attributed the dioritic and xenolithic rocks to assimilation by the granite magma of already consolidated basic rocks similar to the hypersthene-gabbros of the basic mass. However, the basic rocks of the Insch mass have been more fully investigated since the early work on the Insch granitediorite complex (Read, Sadashivaiah, and Haq, 1965; Clarke and Wadsworth, I97o), and it is now recognized that two quite distinct types of hypersthene-gabbro are present. One of these is part of the layered cumulate sequence; the other (granular hypersthene-gabbro) is apparently a separate, later intrusion.

The isotopic and structural age of the Aberdeen Granite

“…Discordant U-Pb systems in zircons from the Aberdeen granite (Pidgeon & Aftalion 1978) show an inherited component, defining an upper intercept of the reverse discordia in excess of 1000 Ma. 6l80 values are around + l 0 per mil relative to SMOW (Kay 1980 (Pidgeon & Aftalion 1978;Hamilton et al 1980;Pankhurst 1974;Busrewil et al 1975;Bell 1968), for example, the Strichen and Longmanhill granites give T:AuR ages of 1406 Ma and 1608 Ma respectively (recalculated from Hamilton et al 1980), and the former has yielded discordant zircons with an upper intercept of c. 2000 Ma. Strontium isotope initial ratios are generally high (see Pankhurst & Sutherland 1982 for summary).…”

Cumulate relations in the ‘Younger Basic’ masses of the Huntly–Portsoy area, Grampian Region

“…The other bodies are smaller and do not have a distinct expression on the aeromagnetic map. All of these cumulate occurrences appear to be separated from each other by areas of mafic rocks showing no obvious cumulate characteristics, with fine-grained, granular norites that often contain country rock xenoliths being particularly abundant, although coarser (2-3 mm) rocks that may be the equivalents of the quartz-biotite-norites of the Boganclogh (Busrewil et al 1973) and Insch (Busrewil et al 1975) masses have also been recorded.…”

Geochemistry of diorites and associated plutonic rocks of SE Jersey, Channel Islands

“…Intermediate plutonic rocks have been ascribed C. H. KEY to a variety of petrogenetic processes including intrusion of a primary dioritic magma (Busrewil et al, 1975), fractional crystallization of basic magma (Perfit et al, 1980;Snoke et al, 1981;Walsh and Clarke, 1982;Whalen, 1985), liquid immiscibility (Vogel and Wilband, 1977;Bender et al, 1982), and mechanisms which involve mixing and hybridization of acid and basic magmas (Vogel, 1982;Vogel et al, 1984;Whalen and Currie, 1984;Mezger et al, 1985). Nockolds (1934;1941) employed contamination and reciprocal reaction where a solid basic rock is metasomatically recrystallized by a later fluid granite to the extent that the products attain chemical, mineralogical and possibly textural characteristics intermediate between those of the parent rocks.…”