D. Barrie Clarke

Origin of chemically zoned and unzoned cordierites from the South Mountain and Musquodoboit Batholiths, Nova Scotia

Textural relations and chemical zoning of cordierites in granites act as sensitive recorders of the conditions of their crystallisation history and underlying magma chamber processes. In this contribution, we present new data on texturally distinct and variably zoned cordierites from the late-Devonian, granitic South Mountain and Musquodoboit Batholiths, and infer the conditions of their formation. Using a combined textural (grain size, grain shape and inclusion relationships) and chemical (major element composition and compositional zoning) classification, we recognise the following six cordierite types: CG1/TT1, anhedral to subhedral macrocrysts with random inclusions and patchy normal zoning; CG2a/TT2, euhedral to subhedral macrocrysts with random inclusions and normal zoning; CG2b/TT2, euhedral to subhedral macrocrysts with random or oriented inclusions, and oscillatory zoning; CG3a/TT3, subhedral to euhedral microcrysts with no inclusions and reverse zoning; CG3b/TT4, euhedral macrocrysts with no inclusions and no zoning; and CG4/TT5, anhedral macrocrysts with random inclusions and normal zoning. The textural criteria suggest that these cordierites formed as a product of cotectic crystallisation from a melt, or as the result of a peritectic reaction involving country-rock material. The combined chemical and textural criteria suggest that: (1) normal zoning results from cotectic crystallisation during cooling, cotectic overgrowths on grains formed in a peritectic reaction with country-rock material, or cation exchange with a fluid; (2) oscillatory zoning results from cotectic crystallisation during variations in X Mg of the silicate melt following magma replenishment; (3) reverse zoning results from crystallisation during pressure quenching; and (4) the unzoned cordierite results from cotectic crystallisation under fluid-rich conditions.

Is stoping a volumetrically significant pluton emplacement process?: Comment

[Glazner and Bartley (2006)][1] have challenged the “large-magma-body-emplaced-by-stoping” model for granite batholiths. Instead, primarily as a consequence of their geochronological data on the Tuolumne Batholith, [Glazner and Bartley (2006)][1], building on [Glazner et al. (2004)][2] and [

Granite-hosted mineral deposits of the New Ross area, South Mountain Batholith, Nova Scotia, Canada: P, T and X constraints of fluids using fluid inclusion thermometry and decrepitate analysis

The most evolved rocks of the South Mountain Batholith host polymetallic (Sn-W-U-Mo-Cu-Mn) mineral deposits in the New Ross area (NRA). This project presents new data (fluid inclusions, stable isotopes, mineral chemistry, and 40Ar/39Ar and 187Re/187Os dating) to define the hydrothermal fluids related to the mineralisation of the NRA, and to create an integrated model for the origin of the mineral deposits. Single-grain Ar/Ar laserprobe ages for white micas from unmineralised and mineralised samples range from 382 to 320 Ma. Molybdenite Re/Os ages on two pegmatite samples are 377 +/- 3 Ma and 371 +/- 3 Ma. The main phases of hydrothermal activity leading to polymetallic mineralisation occurred in close temporal relationship to granite crystallisation at ∼380 Ma, and episodic reheating events occurred at ∼371 Ma and ∼320 Ma, partially resetting the Ar/Ar system. Three distinct fluids in the mineral deposits of the NRA are: (i) a Na/K-rich magmatic fluid; (ii) a Ca-rich metamorphic fluid; and (iii) a meteoric fluid. Mixing of these fluids occurred, implying that the three fluids were contemporaneous. The isotopic compositions (delta 18O, deltaD) of white mica from unmineralised and mineralised samples of the NRA, and deltaD values for fluid inclusion extracts from coexisting quartz, record a transition from a magma-dominated, low water/rock ratio system to a fluid-dominated, higher water/rock ratio system, into which meteoric water infiltrated at the time of SMB crystallisation and greisen/vein formation. Fluctuating pH conditions of hydrothermal fluids at the contact with host granitoid rocks and variations in redox conditions, triggered by incursion of increasing amounts of meteoric fluid along deep-penetrating faults as the system evolved from magmatic to hydrothermal, caused metal deposition. Over ∼10 My, the concurrence of highly evolved granitic melts rich in incompatible elements in the late-stage of granite emplacement with intense fluid circulation, particularly those of meteoric origin, formed the NRA mineral deposits.

Two centuries after Hutton's ‘Theory of the Earth’: the status of granite science

Granite is the final product of the high-temperature, magmatic, predominantly endogenic, chemical differentiation of the earth. Our understanding of the origin and evolution of granitoid rocks comes from a combination of direct observation, analogue experimentation and numerical modelling. A brief historical overview shows an exceptional level of such research activity over the last 50 years. The number and complexity of questions have resulted in both an absolute and a relative growth of the science since the plate tectonic revolution, largely consisting of refining the current magmatic paradigm within its overarching context. Current research activity involves large components of mineralogical–petrological–geochemical and structural–tectonic work, with much lower levels of experimental, geophysical and geochronological investigations. Many important questions concerning the thermal, physical and chemical aspects of the origin and evolution of granites remain. In keeping with the general progress of science, the complexity of the questions, the declining financial support and the revolution in information technology, directions of granite research in the foreseeable future will change from concrete and qualitative to abstract and quantitative, from expensive and active to cheap and armchair, from reductionist to holistic, and from periodic communication to continuous communication.

Multivariate statistical models for granites in terrane analysis: Nova Scotia, Morocco, and Iberia

The Meguma Zone of southern Nova Scotia consists mainly of Cambrian-Ordovician flyschoid metasedimentary rocks and Devonian-Carboniferous peraluminous granitic intrusions. It does not correlate with other lithotectonic zones in the Appalachians and may be a suspect terrane. Studies of the flyschoid rocks suggest that their source lay to the southeast, and a sedimentological case has been made for Morocco as the source region. This study uses discriminant function analysis on whole-rock geochemical data from the granites to test the correlation between Morocco and Iberia. To eliminate the problem of closure, a log ratio transformation is applied to the major-element data, and to approximate normal distributions, a log transformation is applied to the trace-element data. Statistical models show broad geochemical similarities between the peraluminous granites of Nova Scotia, northern Morocco, and Iberia. In particular, several plutons in both Nova Scotia (Sherbrooke, Ellison Lake, Moose Lake) and Morocco (Zaer, Ment, Sebt de Brikiine) are strongly misclassified as Moroccan and Nova Scotian, respectively. Also, a comparison between these North Atlantic peraluminous granites as a group, and a control suite from Australia, shows the differences between the two populations to be considerably larger than the differences between Nova Scotia, Morocco, and Iberia. Peraluminous granites appear to have distinctive geochemical signatures on an orogenic scale.

Calcium-poor pyroxenes in the system CaOMgOAl2O3SiO2FeO2

Abstract A theoretical treatment of the solid solution ranges of pyroxenes in the quaternary system CaOMgOFeOSiO 2 is presented. Experimentally determined phase equilibria and analyses are reported for; protopyroxenes with up to 5.6 wt.% FeO; orthopyroxenes with up to 2.6 wt.% CaO and with levels of Fe 2 O 3 probably approaching 5.0 wt.%; and clinopyroxenes (P2 1 /c, pigeonites) with CaO from 1.4 to 6.2 wt.% and with 2.4 to 31.1 wt.% FeO.

The Port Mouton Shear Zone: intersection of a regional fault with a crystallizing granitoid pluton

Abstract The peraluminous tonalite–monzogranite Port Mouton Pluton is a petrological, geochemical, structural, and geochronological anomaly among the many Late Devonian granitoid intrusions of the Meguma Lithotectonic Zone of southern Nova Scotia. The most remarkable structural feature of this pluton is a 4-km-wide zone of strongly foliated (040/subvertical) monzogranites culminating in a narrow (10–30 m), straight, zone of compositionally banded rocks that extends for at least 3 km along strike. The banded monzogranites consist of alternating melanocratic and leucocratic compositions that are complementary to the overall composition of that part of the pluton, suggesting an origin by mineral–melt and mineral–mineral sorting. Biotite and feldspar are strongly foliated in the plane of the compositional bands. These compositional variations and foliations originated by a process of segregation flow during shearing of the main magma with a crystallinity of 55–75%. Subsequent minor brittle fracturing of feldspars, twinning of microcline, development of blocky sub-grains in quartz, and kinking of micas demonstrate overprinting by a high-temperature deformation straddling the monzogranite solidus. Small folds and late sigmoidal dykes indicate dextral movement on the shear zone. This Port Mouton Shear Zone (PMSZ) is approximately co-linear with the only outcrops of Late Devonian mafic intrusions in the area, two of which are syn-plutonic with well-developed mingling textures in the marginal tonalite of the Port Mouton Pluton. Also closely co-linear with the mafic intrusions are a granitoid dyke that extends well beyond the outer contact of the Port Mouton Pluton, a swarm of large aligned angular xenolithic slabs, a zone of thin wispy schlieren banding, a large Be-bearing pegmatite, and a breccia pipe with abundant garnetiferous metapelitic xenoliths. In various ways, the shear zone may control all of these features. The Port Mouton Shear Zone is parallel to many other NE-trending faults and shear zones in the northern Appalachians, probably related to the docking of the Meguma Zone along the Cobequid–Chedabucto Fault system.

A theory for some pyroxene crystallization trends

Abstract Changes in solid solution limits, which must occur as crystallization proceeds, are used to interpret crystallization trends of pyroxenes from the Bushveld Intrusion, from oxidised and reduced basaltic lavas, and from lunar lavas.

Protoenstatite solid solution in the system CaOMgOAl2O3SiO2

Abstract A model of the solid solution limits of protoenstatite in the system CaOMgOAl 2 O 3 SiO 2 is proposed. Analyses of some experimentally prepared protoenstatite crystals define some of the compositional limits between 1220°C and 1390°C. CaO ranges up to 4.10 wt. % and Al 2 O 3 up to 1.62 wt. %.