Biotite Porphyroblasts Research Articles

Fabrics and Deformation of Archean Metasedimentary Rocks, Ross Lake – Gordon Lake Area, Slave Province, Northwest Territories

Metagreywackes, which lie stratigraphically above metavolcanics fringing a granitic complex (part basement), exhibit late Archean multiphase fold structures. F1 folds are large linear depressions and anticlinal culminations trending partly concordant with margins of the granitic complex. Open to isoclinal F2 folds vary and curve in trend. Though usually near upright, some F2 folds are overturned away from the complex and others towards F1 depressions. F3 folds are small-scale structures not present in most outcrops, but throughout a large region they are accompanied by a steep S3 axial-plane schistosity striking NW to N.Original orientations of an S2 schistosity, axial planar to F2 folds, are preserved as inclusion trails in biotite porphyroblasts. Angular deflections from the trails show that regional horizontal flattening across the S3 schistosity formed the F3 folds and tightened curved trends of F2 folds. Estimates of strain from the fabrics indicate no increase near the complex.As for folds in other Archean terrains, F1 and F2 folds could have developed mainly as gravity structures during diapiric uprise of a granitic basement. In contrast, the F3 structures may reflect horizontal movements of early crustal plates.

Preliminary Petrographic, Chemical, and Age Data on Some Intrusive and Associated Contact Metamorphic Rocks, Pioneer Mountains, Southwestern Montana

Petrographic and chemical data on five types of intrusive rocks of the so-called Pioneer batholith in the northeastern Pioneer Mountains of southwest Montana show that the rocks range from quartz diorite to granite. The volumetrically most important body is a coarse-grained biotite-hornblende “granite” that superficially resembles the Butte Quartz Monzonite of the Boulder batholith, some 60 km to the northeast. Their K 2 O content puts the rocks into Tilling9s “sodic series” of the Boulder batholith. K-Ar dating on biotite and hornblende shows that the date of intrusion of all but the quartz diorite is about 70 m.y. The hornblende age of the quartz diorite indicates that it was intruded 76.5 m.y. ago. Its biotite age of 70.5 m.y. agrees with those of the younger intrusions. High-grade contact metamorphic rocks of the Silver Hill Formation (Cambrian) at one locality yielded the same 70-m.y. age on biotite porphyroblasts. These ages compare closely with those of the Boulder and Philipsburg batholiths and show the same tendency for the less mafic rocks to be younger.

On the Metamorphic Rocks of Kung-Chang-Lin district, South Manchuria

1. The metamorphic rocks of Kung-Chang-Lin district belong to the amphibolite to epidote-amphibolite facies. 2. Although existing with typical epidote-amphibolites, amphiboleschists, which are composed of almost exclusively amphibole, are appear to be in lower metamorphic grade than the former. 3. Leptitic rocks, originally thought to be composed of plagioclasequartz-chlorite, or plagioclase-quartz-biotite, were produced by the pneumatolytic action of a subjacent rising granitic magma. These rocks exist as a continuous rock-bed intercalated between two banded iron-ore beds and their mineral compositions are varied locally. 4. Porphyroblasts of staurolite, almandine and biotite were developed in staurolite-almandine-mica schists by the metamorphic diffusion under distinct stress condition. 5. The pneumatolytic metamorphism related to microcline-granite (so-called Kung-Chang-Lin granite) occured before the complete consolidation of its boundary facies. This view is favourable by the developement of a quartz schist, which is distinctly of pneumatolytie origin, along the boundary of banded iron-ore and the granite.