The Ghirian volcanic province comprises three distinct units. An early (50-55 my.) phase of extensive plateau lava (basaltic hawaiite) eruption was followed by the emplacement of small phonolite domes ( N 40 m.y.). After a long period of inactivity, a rejuvenation phase (< 12 m.y.ago) led to the formation of scattered minor volcanic centres, generally of basanitic composition. and carrying lherzolite fragments. 39 new analyses illustrate the distinctive chemical characteristics of each episode. Differences in volume and composition between early and late basic volcanicity can be related to regional episodes of strong mantle convection in the early Tertiary and weak convection in the late Tertiary. The phonolites may be fractionates of an intervening injection of basic magma trapped at moderate depth. Continuing structural control was exerted on a regional scale by the zone of uplift and fracturing connecting the Ghirian area with Tibesti.

SYNOPSISThe Boganclogh intrusion, which appears to be a westerly extension of the Insch mass, is chemically and petrologically subdivided into three principal units which occupy roughly east-west belts.

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.

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