Porphyritic Biotite Granite Research Articles

Tin granites of Seward Peninsula, Alaska

Seven granite plutons, spatially and genetically related to tin metalization, are exposed in a 170-km-long belt across northwestern Seward Peninsula, Alaska. These plutons are cupolas and epizonal composite stocks that consist of several textural varieties of biotite granite, including medium- to coarse-grained seriate biotite granite, porphyritic biotite granite with an aplitic groundmass, and fine- to medium-grained equigranular biotite granite. The common accessory minerals are fluorite, allanite, apatite, and zircon. Other accessory minerals that are locally present include tourmaline, sphene, opaque oxide minerals, and late-forming (deuteric) muscovite and chlorite. The granites range in major-element contents as follows: SiO 2 , 72.5% to 76.6%; A1 2 O 3 , 12.7% to 14.3%; Na 2 O, 2.9% to 4.0%; K 2 O, 3.9% to 5.6%; and CaO, 0.6% to 1.2%. The sum of FeO + Fe 2 O 3 + MgO ranges from 0.3% to 2.4%; and the K 2 O to Na 2 O ratio from 1.1 to 1.8. The 0.1% to 0.9% F and 0.01% to 0.2% Cl reflect the over-all volatile-rich nature of the granites. The granites contain average or below-average concentrations of Co, Sc, Cr, and Zn, and generally above-average to distinctly high concentrations of Th, U, Hf, and Ta. The large cations emphasize the evolved nature of the granites; the Rb/Sr ratio is as high as 90 in some samples. Initial 87 Sr/ 86 Sr ratios range from 0.708 to as high as 0.720. The three Rb-Sr isochrons defined by the data agree with K-Ar age determinations and show that the stocks were emplaced during the Late Cretaceous, between about 70 and 80 m.y. ago. The field, petrologic, and geochemical data indicate that the plutons had a multistage origin that involved large-scale melting of sialic crust, emplacement of magmas derived from batholithic fractionation at depth, and subsequent evolution of these magmas to generate small volumes of more highly evolved residual magmas. Although evolution of the granite complexes was largely governed by crystal-melt fractionation, some minor-element variations in the highly evolved granites cannot be explained by this process. For example, the distribution of rubidium and the light rare-earths appears to have been influenced by volatile depletion at the final stages of crystallization. The field data, petrologic data, and variation trends, such as distinct shifts toward higher albite contents in the residual granites, suggest that the coexistence of a volatile phase was important in their evolution. These results require that models seeking to explain compositional gradients in high-level granite (rhyolite) systems fully consider the role of a coexisting volatile phase.

Geologic setting and genesis of cassiterite-sulfide mineralization at Renison Bell, western Tasmania

Cassiterite mineralization at Renison Bell is spatially and temporally related to the Upper Devonian Pine Hill Granite and occurs in dolomites and associated clastic sediments of probable Cambrian age. The sedimentary succession consists of an older siliceous clastic and carbonate sequence (the Success Creek Group) overlain by, and in faulted contact with, a predominantly volcaniclastic sequence (the Crimson Creek Formation). The Success Creek Group includes thinly bedded quartzites, siltstones, shales, and dolostones of probable shallow marine origin, and the Crimson Creek Formation consists of siltstones and volcaniclastic graywackes of basaltic composition. The Success Creek and Crimson Creek rocks are separated by the Red Rock sequence, consisting of polymict conglomerates, tuffs, cherts, and carbonate rocks. Mafic intrusives in the Crimson Creek Formation are chemically similar to the enclosing sediments. Carbonate beds in the Success Creek Group and the Red Rock sequence consist principally of highly ordered ferroan dolomite, and their textural, mineralogical, and stable isotopic features suggest they are diagenetically dolomitized marine limestones.The Pine Hill Granite is a porphyritic biotite granite which has been locally altered to greisen and muscovitic assemblages carrying minor cassiterite. Adjacent rocks have been thermally metamorphosed to hornblende-hornfels facies assemblages.Cassiterite mineralization occurs in stratabound replacement bodies in dolomite beds in the Success Creek Group and Red Rock sequence, in veins with variable wall-rock replacement in major fault zones and to a minor extent in recrystallized sandstone. The generalized paragenetic sequence in the Renison mine is:stage 1: cassiterite + silicates--tentatively inferred as an early stage of replacement ore formation;stage 2: cassiterite + pyrrhotite + arsenopyrite + silicates + minor sulfides and iron oxides, etc.--the main stage of mineralization, associated with sideritic alteration of dolomites and tourmalinization of clastic sediments;stage 3: cassiterite + pyrrhotite + arsenopyrite + silicates + minor sulfides--forming veins in major fault zones;stage 4: minor veining by sphalerite + galena + silicates + carbonates + or - fluorite; and stage 5: vug-filling sequence of carbonates, quartz, fluorite, and sulfides.Temperatures of about 350 degrees and 300 degrees C are inferred for stages 1 to 3 and stage 4, respectively, from fluid inclusion, mineralogical, and stable isotope data. Temperatures during stage 5 mineralization were probably in the range 200 degrees to 150 degrees C and were probably lower during stage 6 mineralization.Bulk fluid inclusion analyses indicate that fluids during stages 1 to 4 were Na-K-Cl brines with a total salt concentration of about 2 molal and an Na/K ratio of about 7. Variable Ca and Mg concentrations may be partly due to contamination during sample leaching. Fluids were gas poor with average gas/water molal ratios of about 2 X 10 (super -2) and CO 2 /CH 4 ratios near 1. Nitrogen and argon were detected only in some samples. Stage 5 fluids were more saline, with average salt concentrations of about 5 molal and Na/K ratios of about 20. Formation of cassiterite-bearing stages took place at about 350 degrees C at low f (sub O 2 ) (about 10 (super -31 5) atm), low f (sub S 2 ) (10 (super -11) -10 (super -12 3) atm), and low pH (3.9-5.4) in the reduced sulfur field. The major control on ore deposition was probably an increase in pH within the indicated range due to dolomite replacement. Calculated stabilities of stannous chloride complexes indicate that tens to hundreds of parts per million of tin may be transported in this form in low f (sub O 2 ) -low pH conditions, and may be precipitated as cassiterite from sulfur-poor solutions by mechanisms which would cause precipitation of the accompanying sulfides.Sphalerite-galena mineralization took place at low f (sub O 2 ) (about 10 (super -34 5) atm), low f (sub S 2 ) (10 (super -10 2) -10 (super -10 9) atm), and low pH (3.9-5.5) in the reduced sulfur field.Oxygen and hydrogen isotope ratios of the mineralizing fluids, calculated from analyses of gangue minerals and water extracted from fluid inclusions, suggest that the stage 1 to 3 fluids were an exsolved magmatic aqueous phase. Water from a different source, probably heated contemporary ground water, is inferred for fluids in later stages. Some mixing of fluid types may have occurred during stage 3. Calculated delta 34 S (sub Sigma S) values for stages 1 to 3 and stage 4 fluids average 6.0 per mil and 5.3%, respectively, and are consistent with the Pine Hill Granite being the major or sole source of sulfur for the fluids.

The Gander Lake and Davidsville Groups of Northeastern Newfoundland: New Data and Geotectonic Implications: Discussion

A large number of papers on Newfoundland geology have appeared lately, many of them stimulated by the publication of the plate tectonics hypothesis some four years ago. Kennedy and McGonigal (197'2) have recently added another, this one an interpretive paper on the tectonic events in northeastern Newfoundland. It should surprise no one even slightly familiar with the region that interpretational conflicts might arise. for rocks are very poorly exposed in northeastern Newfoundland (<5 5% outcrop) and much extrapolation, speculation, and imagination are generally required to arrive at any interpretations. Crucial to many of the authors' conclusions is the age of the leucocratic granite. This they regard as intrusive into their metasedimentary terrane (i.e. into the lower unit of the Gander Lake Group1 ), and interpret it as pre-Middle Ordovician in age. Exposures of this peculiar garnetiferous, pcgmatitic, sodic, white muscovite granite occur with a northeasterly trend and at irregular intervals along the entire eastern side of the Gander River ultrabasic belt. With few exceptions they lie west of the migmatites and porphyritic reddish biotite granites in northeastern Newfoundland. Two such bodies of lcucocratic granite south of Gander Lake have 'Throughout this discussion 1 have used the term Gander Lake Group as it was originally defined (Jenness 1958, 1963), for reasons that will be apparent later in the discussion.

Investigation of Initial Sr87 / Sr86 Ratios in the Sierra Nevada Plutonic Province

One to three whole-rock samples from each of more than a dozen discrete plutonic intrusions in the east-central Sierra Nevada batholith have been analyzed for Sr 87 /Sr 86 and Rb/Sr ratios to obtain information on initial Sr 87 abundances. The initial Sr 87 /Sr 86 ratios in the rock magmas forming this province appear to have been in the range 0.7073 ± .0010 in the majority of cases. This range is definitely higher than that found for modern alkali-type and tholeiite-type basalt magmas of oceanic regions, which commonly range between 0.703 and 0.705. However, it is much lower than the average Sr 87 /Sr 86 ratios found in Precambrian sialic regions which range from 0.71 to 0.73. It seems clear therefore that the Sierra Nevada magmas were not derived solely either from the typical source regions of oceanic basalt or from the melting of ancient crustal sial. It is possible that these magmas represent a mixture of oceanic basalt and crustal sial, as would be the case of anatexis in a geosyncline containing much volcanic material of fairly recent origin and some terrigenous sialic detritus. They may instead be of mantle derivation with admixtures of crustal material assimilated during their rise. The whole-rock Rb-Sr age results derivec from the study indicate that the Lamarck and Mount Givens Granodiorites and the alaskite of Evolution Basin and porphyritic biotite granite of Dinkey Lakes form a younger group of intrusive rocks of 90 ± 10 m.y. Although the sampling was not designed for isochron age studies, it appears that most of the remaining rock units are considerably older.

POST-MAGMATIC HIGH TEMPERATURE METASOMATIC PROCESSES IN GRANITIC ROCKS

Microclinization of granites originally enriched in plagioclase forms 1) porphyritic (porphyroblastic) biotite granites; 2) coarse- to medium-grained (even-grained) biotite-, two-mica, and muscovite granites; and 3) pegmatitic granites. Additional post magmatic metasomatic processes effecting granites are early albitization, greisenization, late albitization, and late microclinization. Maximum relative activity of cations in the evolution of post magmatic solutions may be arranged in the following series: K-Na-Si-Al-Na-K or else (with greisenization stage undeveloped) K-Na-K. This series corresponds to the concept of acidity-alkalinity of high temperature post-magmatic solutions and reflects the changes in their acidity at the various stages of metasomatism.--R. M. Hutchinson.

Bedrock Geology of the Thiel Mountains, Antarctica

Cordierite-bearing, hyper-sthene-quartz monzonite porphyry, the most widespread rock unit, is intruded by biotite granite and porphyritic biotite granite. Sedimentary and metasedimentary rocks, mainly quartzites and argillites, have been metamorphosed locally to hornfels and have been involved in high-angle faulting. Shear zones are common in the plutonic rocks.

THE STRUCTURAL FEATURES AND INCLUSIONS OF THE CAPE PENINSULA GRANITE

Abstract The coarsely porphyritic biotite-granite of the Cape pluton, intrusive into the steeply dipping Malmesbury Series, extends the full length of the Peninsula. Structural features are feebly developed, primary flow structures being absent. The most prominent joint directions strike WNW.-ESE. parallel to the direction of stretching. They have been classified as longitudinal joints. Transverse NNW.-SSE. and NNE.-SSW. trending fractures are regarded as cross-joint directions. Secondary structures are undoubtedly present, but the primary age of these fracture systems is established by the fact that aplites follow them. The Malmesbury Series in this area consists of psammitic strata with thinner pelitic intercalations. Both arenaceous and argillaceous strata have been converted into hornfels in the neighbourhood of the granite. Xenoliths of the Malmesbury sediments included in the granite show every gradation between normal hornfels and completely granitized fine-grained leucocratic granitoid rocks. Biot...