Michael P. Atherton

The variation in garnet, biotite and chlorite composition in medium grade pelitic rocks from the Dalradian, Scotland, with particular reference to the zonation in garnet

Garnet, biotite and host rock have been analysed along a traverse from the garnet isograd to the kyanite zone in the Dalradian of Central Perthshire, Scotland. FeO and MgO increase and MnO and CaO decrease in the garnet with increasing grade. Microprobe analyses of the garnets reveal zoning, which indicates that a garnet crystal as a whole does not equilibrate with the matrix during growth. Coexisting biotite varies in composition as a result of the abstraction of MnO, FeO etc. from the rook by the growing garnet, i.e. the mg/mg + fe ratio increases with grade. The microprobe analyses also reveal the size of the system from which garnet abstracted material varied from 0.100 to 2.000 g and the nucleation was frequently “instantaneous”. It also reveals the equilibrium or non-equilibrium nature of the assemblage, and explains the variation in garnet composition with grade in terms of a segregation model with a changing distribution coefficient. Primary chlorite was analysed from rocks near to the garnet isograd containing garnet and biotite. It has a similar mg/mg + fe value to the coexisting biotite. The results show that the three phase field defining the garnet isograd moves towards the mg corner with increasing grade. The higher grade fields lie to the mg rich side of the three phase field so that the sequence of mineral assemblages across the Barrovian zones in Perthshire, from the garnet isograd to the kyanite zone, can be summarized and displayed on a phase diagram.

Granitoid emplacement and deformation along a major crustal lineament: The Cordillera Blanca, Peru

Abstract The Cordillera Blanca Batholith and ignimbrites of the intermountain Callejon de Huaylas basin represent the last magmatic event in the Andean cycle of central-northern Peru. This most spectacular example of an extensional regime is characterised by acid magmatism spanning some 10 Ma, with both magma emplacement and subsequent deformation being fundamentally controlled by a ca. 300-km lineament known as the Cordillera Blanca fault complex. The western margin of the batholith has been strongly deformed by the Cordillera Blanca fault, resulting in a series of ductile to brittle structures that increase with intensity away from the relatively undeformed core towards the margin. The late-stage brittle fabrics are coeval with extensional structures in the Callejon de Huaylas basin, and relate to uplift and extension along the fault zone. The more ductile fabrics, as indicated by combined K-Ar and 40 Ar- 39 Ar mineral dates, formed earlier in the intrusion history, and are consistent with emplacement into an active dextral (strike-slip) shear zone. The rapid uplift and high erosion rates seen from the Late Miocene onwards and the current state of stress in this region of thickened crust are due to gravitational instability (body forces) set up at high topographical levels, and buoyancy forces resulting from the intrusion of young, acid magmas at high crustal levels.

Quantitative Modeling of Granitic Melt Generation and Segregation in the Continental Crust

[1] We present a new quantitative model of granitic (in a broad sense) melt generation and segregation within the continental crust. We assume that melt generation is caused by the intrusion of hot, mantle-derived basalt, and that segregation occurs by buoyancy-driven flow along grain edges coupled with compaction of the partially molten source rock. We solve numerically the coupled equations governing heating, melting, and melt migration in the source rock, and cooling and crystallization in the underlying heat source. Our results demonstrate that the spatial distribution and composition of the melt depends upon the relative upward transport rates of heat and melt. If melt transport occurs more quickly than heat transport, then melt accumulates near the top of the source region, until the rock matrix disaggregates and a mobile magma forms. As the melt migrates upward, its composition changes to resemble a smaller degree of melting of the source rock, because it thermodynamically equilibrates with rock at progressively lower temperatures. We demonstrate that this process of buoyancy-driven compaction coupled with local thermodynamic equilibration can yield large volumes of mobile granitic magma from basaltic and meta-basaltic (amphibolitic) protoliths over timescales ranging from ∼4000 years to ∼10 Myr. The thickness of basaltic magma required as a heat source ranges from ∼40 m to ∼3 km, which requires that the magma is emplaced over time as a series of sills, concurrent with melt segregation. These findings differ from those of previous studies, which have suggested that compaction operates too slowly to yield large volumes of segregated granitic melt.

The Tak Batholith, Thailand: the evolution of contrasting granite types and implications for tectonic setting

Abstract The Tak Batholith, Thailand, is made up of four zoned plutons, the youngest of which is 210 Ma old. The composition of the plutons changes from granodioritic in the oldest, through monzonitic and monzogranitic to syenogranitic in the youngest. The earliest pluton (Eastern) shows an extended compositional range and is calc-alkaline-granodioritic, medium K in the sense of Lameyre and Bowden. It is similar to classic Andean Cordilleran Batholith plutons. The three later plutons (Western, Mae Salit and Tak), the first of which also shows an extended compositional range, are all calc-alkaline- monzonitic, high K plutons characterised by the early precipitation of U, Th, REE rich accessory minerals and K-feldspar. In this sense they are very different from the Eastern pluton, in which the dominant early felsic phase crystallising is plagioclase and accessory mineral precipitation tends to be late. The changes in major and trace element composition of the plutons with time are similar to that seen in an increasing K volcanic series at an active continental margin. Indeed, the final member is chemically similar to shoshonitic volcanic rocks and by analogy the plutonic sequence is considered to be due to a change from subduction to strike-slip and then uplift, which brought mantle of different composition into the source region beneath the Tak Batholith. In such situations simple classifications relating a batholith to end-member source and geotectonic setting may be misleading.

Shape and intrusion style of the Coastal Batholith, Peru

Abstract The Coastal Batholith of Peru extends over 1600 km parallel to the coast along the Andean trend. Gravity profiles on three traverses across the batholith indicate the geometry is essentially that of a flat slab with average thickness from 2.0–3.2 km, and a thick root 4–10 km wide to the west. Granitic material does not extend to depths greater than 3 km below sea level datum. This study supports recent gravity work which indicates plutons are commonly thin, 5 km or less in thickness. Detailed mapping in the Lima segment of the Coastal Batholith reveals thin plutons where space was made dominantly by downward displacement via floor depression. However, early roof uplift also created some space. Stoping occurs but is not a major space maker. Floor depression may be modelled by cantilever or piston mechanisms and although the strong marginal deformation with mylonites, tuffisites, microbreccia, faults and shear zones suggests the piston model best describes the mechanism of emplacement of much of the Coastal Batholith some space was probably made by a cantilever mechanism. In brief, space making processes involved early roof uplift and regional doming, then floor depression mainly by piston and probably subsidiary cantilever mechanisms and, finally, local stoping producing the cut-out rectilinear nature of the batholith. The Coastal Batholith formed on shallow partial melting of hydrous basaltic marginal basin rocks between 5 and 10 km. Floor depression occurred as the crustal column foundered into an actively deflating layer of partial melt. This is an efficient space making process and is limited here to shallow levels of the upper crust only. The melts ascended to within 2 or 3 km of the surface, up dyke-like conduits then spread horizontally to form tabular plutons.

The description of the primary textures of “Cordilleran” granitic rocks

Variation in the primary textures of “Cordilleran” granitic rocks is described relative to three identifiable stages of the crystallisation interval; namely: (1) crystallisation in suspension; (2) growth of a touching crystal framework; (3) interstitial crystallisation. Crystals that initially grow in isolation will start to impinge and form small clusters as crystallisation proceeds and the volume of solid material increases, eventually forming a continuous interconnected crystal framework. Subsequent crystallisation involves solidification of the melt occupying the interstices of the framework, and therefore shows similarities to the way in which the porosity occludes in sedimentary systems. A case study of textural development in Cordilleran granitic rocks from the zoned Linga superunit of the Peruvian Coastal Batholith, reveals that compositional zonation from granodiorite through to syenogranite is accompanied by a systematic variation in the textures, specifically those of the three felsic phases (plagioclase, quartz and alkali feldspar). Plagioclase was the first phase to appear on the liquidus, and was joined by the other two phases as crystallisation proceeded and the melt evolved. The melt fraction at which quartz and alkali feldspar started to crystallise influenced the early growth of plagioclase, and the way in which the texture developed through each stage of the crystallisation interval. The geometry of plagioclase progressively changes from a touching framework of crystals in the granodiorite, to small aggregates or isolated crystals suspended in an equant mosaic of the other felsic phases in the syenogranite. This variation can be explained by an earlier evolution of the melt to the cotectic (i.e. at higher melt fractions) as the rocks become more acidic, and hence a greater contribution of alkali feldspar and quartz to the growth of the framework at the expense of plagioclase and the mafic phases. Textural observations are comparable to the crystallisation pathways of the felsic phases modelled in the quaternary An-Ab-Or-Qz system from the bulk compositions. All compositions lie in the plagioclase volume, and evolved to three-phase saturation on the cotectic via either the quartz/plagioclase divariant surface (granodiorites) or the alkali feldspar/plagioclase divariant surface (monzogranite and syenogranite).

Cretaceous-Tertiary volcanism and syn-subduction crustal extension in northern central Peru

Abstract Although subduction of the Nazca plate beneath the Peruvian continental margin has been in progress for nearly 200 Ma, the Andean cycle in western Peru is dominated by periods of contemporaneous rifting and large scale crustal extension. It began with the formation of the west Peruvian trough (WPT), a major depositional structure and one of a series of ‘marginal’ basins that developed along the entire Pacific coast of South America during the Cretaceous. The western element of the WPT north of Lima is the Albian Casma basin, composed of nearly 9000 m of mostly basaltic to intermediate volcanic rocks and minor intrusives. Facies analysis of the basinal fill is consistent with a spreading system in a relatively isolated, deep-sea environment. The Tertiary is marked by two major episodes of extensional, subaerial volcanism: the c. 53-15 Ma Calipuy Group, followed in the Late Miocene-Pliocene (c. 7.6–4.65 Ma) by the more easterly Yungay volcanic rocks. Although both were extruded during periods of crustal extension, the prevailing tectonic regimes differ in detail. Thus the acid-intermediate Calipuy Group was extruded from fissure-type volcanoes over a long period with only minor plutonic activity, whereas the more easterly Yungay volcanic rocks, located furthest from the present trench, are closely associated with major batholith intrusion within a strike-slip transtensional system. Although the volcanic rocks of the Casma-Calipuy-Yungay groups are dominantly ‘calc-alkaline’ in the classic sense, the role of extension has clearly been fundamental in their genesis. In contrast, their relation to contemporaneous subduction is less clear.

Integrated chemistry, textures, phase relations and modelling of a composite granodioritic-monzonitic batholith, Tak, Thailand

Abstract The Tak Batholith, Thailand, is important as it marks the boundary between the two major belts in SE Asia, namely the Eastern and Central belts, defined on granitoid type and geotectonic setting. It is a composite batholith made up of an early calc-alkaline “granodioritic” pluton and three later calc-alkaline “monzonitic” (or sub-alkaline potassic) plutons. The two types have different mineralogy, textures, chemistry and evolution paths, although the major phase petrology is the same. The “monzonitic” plutons, in contrast to the “granodioritic” pluton, are characterised by early precipitation of K-feldspar and accessory minerals such as sphene, allanite, apatite, zircon and magnetite, which means that the REE, Ba, U and Th are depleted in the evolving magmas of the “monzonitic” suites, but are enriched in the “granodioritic” magmas. The associated porphyries indicate the liquidus mineralogy and show that all the magmas started to crystalline in the plagioclase volume of the AbAnOrQz system, with the “monzonitic” magmas quickly reaching the plagioclase/K-feldspar surface while the granodioritic magma had extended crystallisation before reaching the plagioclase-quartz surface. All the magmas finally crystallised on the univariant cotectic. Element modelling using major, trace and REE data supports the differences in the two contrasting lineages, emphasising the importance of early K-feldspar and accessory mineral crystallisation in the evolution of the “monzonitic” magmas. The lack of mineralisation in the Tak Batholith, especially in the “monzonitic” plutons, which have high K, U and Th contents, i.e. they are high heat production (HHP) granites, is intriguing and a common feature of this lineage. Stabilisation of these elements in resistant accessory minerals during the early crystallisation history of these magmas is considered important. Finally, as detailed investigations of granite belts continue, the composite nature of some batholiths indicates that simple classifications, which do not account for any within batholith variation, may be at the least misleading in describing geotectonic setting.