This review concerns intrusive magmatism related to the Cadomian orogeny in the northeastern part of the Armorican Massif of France. The Cadomian orogeny is interpreted to represent tectono-thermal activity in a continental margin above a subduction zone. The North Armorican Shear Zone (NASZ) separates two major Cadomian terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The amalgamation of three terranes within the NACT (the St Brieuc Terrane (SBT), the St Malo Terrane (SMT) and the Mancellian Terrane (MT)) around 540 Ma by sinistral transpression is a key element in the Cadomian orogeny. Cadomian plutonic complexes intrude thc c. 2000 Ma Icartian Gneisses and form the local basement to, and are emplaced into the Brioverian succession. Cadomian magmatism spans approximately 275 Ma, from c. 700 Ma to c. 425 Ma.Early Cadomian foliated plutonic complexes occur within the Guernsey-La Hague structural block of the SBT; they represent volcanic arc granites derived in a subduction zone environment. Late Cadomian post-tectonic plutonic complexes within the GuernseyLa Hague structural block were emplaced c. 500 Ma, exhibit a variety of 'mixed magma' features and have a normal to mature continental arc geochemistry. To the southeast, the Jersey structural block within the SBT reprcsents a higher structural level as well as a greater distance away from the probable trench site. Here, thc late Cadomian posttectonic plutonic complexes were emplaced during Cambrian and Ordovician-Silurian times and represent mature continental arc activity, probably associated with the decay of the Cadomian subduction zonc systcm aftcr transprcssional terrane accretion.The St Malo Terrane, to thc southcast, represents an inverted intra-or behind-arc basin. The migmatite belts within this terranc developcd by crustal anatexis. Emplacement of homogeneous diatexites and leucogranites synkinematically with sinistral strike-slip movement along shear belts gencrated by the transpressive terrane accretion allows this cvent to be dated at c. 540 Ma.The Mancellian Terrane, further to the southeast, comprises low-grade Brioverian metasediments into which has been emplaced the Mancellian Batholith, a late Cadomian post-tectonic series of predominantly granite complexes. These complexes have the geochemical features of volcanic arc granites.Overall, Cadomian magmatism rcpresents c. 275 Ma of plutonic activity broadly related to subduction beneath a continental margin. The early part of the Cadomian orogenic cycle includes volcanic-arc plutonic suites and basaltic volcanic rocks with associated feeder dykes. The peak of the Cadomian orogeny involves intra-or behind-arc basin inversion with concomitant mid-crustal anatexis and sinistrally transpressive terrane accretion c. 540 Ma. The late evolution begins with andesite-rhyolite volcanism and calcalkalinc plutonism occurs over a period of c. 100 Ma during thc decay of the Cadomian subduction zone system.

No abstract

SynopisA number of diorite complexes occur within the Channel Islands region, notably on Jersey, Alderney, and particularly Guernsey. Much of northern Guernsey is made up of the largest of these complexes (fig. S1), the Bordeaux diorite. In the north-western part of this diorite, around Chouet, a complicated association of plutonic rocks occurs. Although the field relationships in this area are sometimes difficult to interpret—this is often the case in diorite complexes—three separate groups of rocks may be distinguished within the association: a diorite group; a granodiorite group; and an inhomogeneous suite of rocks (fig. S2).The widespread diorite group consists predominantly of an even-grained diorite, which is relatively homogeneous but which occasionally grades into an acicular diorite, the latter often containing pods and veins of appinite. The granodiorite group is the least common, occurring as bodies which are interpreted as intrusive sheets and bosses within the even-grained diorite, but occurring as angular blocks within the inhomogeneous suite of rocks. The granodiorite invariably contains rounded diorite xenoliths. The inhomogeneous suite consists of a variety of rocks from patchy, dark diorite, through quartz diorite to tonalite. Commonly, these rock types are intimately associated, often showing gradational contacts with each other and frequently with the more basic portions occurring as ‘xenolithic’ material within the more acidic portions. At contacts between the inhomogeneous suite and the even-grained diorite certain features (e.g. lobate margins and pipe-like structures) indicate that the diorite must have been close to its solidus temperature at the time of emplacement of the inhomogeneous suite. The field relationships between the three groups are interpreted as indicating that the diorite group was emplaced first, followed by the granodiorite group, with both of these clearly pre-dating the inhomogeneous suite.Fifty-nine specimens, chosen to give a representative sample of each of the three rock groups, have been analysed for major, minor, and a selection of trace elements and thirteen of these specimens have been analysed for REE. The chemistry of the analysed rocks confirms the division into three groups, with each group showing distinctive characteristics. Furthermore, chemical plots (e.g. Al2O3, P2O5, Cr, and Ni v. SiO2) show discontinuities and areas of overlap between each group which cannot be explained within the constraints of a single genetic model relating the three groups to each other. This argument is particularly strong for the relationship between the diorite group, which spans the range 50 to 59% SiO2, and the inhomogeneous suite, spanning the range 53 to 68% SiO2. In the area of overlap (53 to 59% SiO2) the two groups are geochemically different. Therefore, for a variety of reasons, including emplacement order, the geochemical characteristics of the groups and the lithological inhomogeneity which is associated only with the chemically intermediate members of the association (the inhomogeneous suite), three quite different and genetically unrelated liquids are required to generate the three groups of rocks.The even-grained diorite shows chemical variation (e.g. with increasing SiO2, decreasing Al2O3, MgO, CaO, Sc, V, Cr, and Ni, and increasing Na2O, La, Nd, and Y) consistent with amphibole + plagioclase fractionation up to 55% SiO2. At 55% SiO2 several elements show a change of slope (e.g. FeO + Fe2O3, TiO2, Rb, Ba, and Zr) indicating the introduction of biotite as a fractionating phase. Increasing total REE content with increasing SiO2 throughout the even-grained diorite supports the contention that amphibole is an important fractionating phase. The higher TiO2, P2O5, Sr, La, Ce, Nd, and Y contents and negligible Cr and Ni contents of the acicular diorite suggest an origin by delayed crystallization of volatile-enriched portions of the diorite group magma.The granodiorite group shows little geochemical variation. Members of this group contain detectable amounts of Cr and Ni, unlike virtually all members of the inhomogeneous suite. For this reason, and because of the field relationships, the granodiorite is considered to be genetically unrelated to members of the inhomogeneous suite and a separate liquid is thus required for its genesis. This liquid may have been the fractionated derivative of some other magma (though if this is so the ‘parent’ is entirely unrepresented at the present erosion level) or it may represent a direct crustal melt. Diorite xenoliths within the granodiorite are chemically similar to the even-grained diorite.Despite the lithological complexity of the inhomogeneous suite, its geochemical unity is clearly established in that, for instance, virtually none of the members of the suite (including even the most SiO2-poor) contain detectable Cr and Ni. Moreover, geochemical variation within the group is rational (with the possible exceptions of Sr, Zr, and Ba) and may be explained in terms of a crystal fractionation model. However, the fractionation must have acted on a liquid itself unrelated to either the diorite or granodiorite group magmas. An additional complication is that later derivative liquids intrude into and partly digest earlier-formed semi-solids of the suite to produce much of the observed inhomogeneity. The phases which have controlled fractionation within the suite include plagioclase (established petrographically as well as geochemically) and hornblende. The role of apatite is uncertain. The fractionation of hornblende is particularly useful in explaining the change in REE contents within the inhomogeneous suite. Total REE contents increase from the dark diorite to the quartz diorite, but decrease from the quartz diorite to the tonalite with concomitant relative HREE depletion. This is taken to be a reflection of the changing hornblende/liquid partition coefficients for REE with increasing SiO2, which are less than one for liquids of basaltic and andesitic composition but greater than one for liquids of dacitic composition.

The Northern Igneous Complex of Guernsey consists of four intrusive bodies,

In northwest Guernsey, excellently exposed sections of coast reveal a wide variety of diorites and associated rocks which are particularly instructive in understanding field relationships in diorite complexes generally. In this area, three groups of rocks may be distinguished. A Diorite Group consists of even grained diorite, acicular diorite and appinite, believed to be related to each other by fractionation. This group was intruded by a Granodiorite Group which contains diorite xenoliths. In turn, both of these were intruded by an Inhomogeneous Suite of rocks varying from meladiorite through to tonalite. Field evidence and geochemistry show that this last, highly variable suite of rocks was formed by the unusual process of fractional crystallization coupled with disruption and partial redigestion of early solid fractionates by derivative liquids, leading to the marked inhomogeneity. Such a mechanism may be more generally applicable but clear evidence for it seems to be restricted to rocks of dioritic composition. The margins of the Inhomogeneous Suite usually consist of tonalite which has piped into the surrounding diorites of the Diorite Group, and the two rock groups are thought to have been at least partially liquid at the same time. The exact shape of pipes may prove to be a useful indicator of physical conditions at the time of magma emplacement in this and other intimate associations of magmas of contrasted composition. Although the Diorite Group was emplaced first, dark (apparently chilled) margins sometimes occur in diorites of this Group where they are in contact with the later Inhomogeneous Suite. In such intimate associations as this, dark margins must be interpreted with extreme caution.