Crystal transfer from magma to wallrock during syntectonic granite emplacement

Field, microstructural and mineral compositional evidence from the Mesoproterozoic K-feldspar megacrystic Red Granite at Oncócua Platform (southwestern Angola) is consistent with crystal transfer from magma to wallrock during syntectonic intrusion. K-feldspar megacrystic Red Granite intruded during folding of wallrock tonalite. Enclaves of the wallrock tonalite are elongated parallel to Red Granite intrusive contacts, a K-feldspar megacryst and hornblende defined magmatic foliation, and a gneissosity in the Red Granite. Stromatic layering present in the tonalite is crosscut by the Red Granite intrusive contacts or is isoclinally folded with fold axial planes and hinges filled with Red Granite. K-feldspar megacryst clusters and curved grain boundaries (i.e., contact melting), as well as thin Red Granite fold axial planar sheets containing K-feldspar megacrysts that are typically wider than the sheets themselves, are all consistent with melt loss and crystal accumulation during solidification. The wallrock tonalite also hosts K-feldspar megacrysts and hornblende phenocrysts that are interpreted to be the same population to those in Red Granite, on the basis of their size, shape, nature of inclusions, compositions, and compositional zoning. We propose that these crystals were transferred from the intrusive Red Granite magma to the wallrock tonalite via magmatic conduits that subsequently collapsed due to external stresses, leaving behind the larger crystals. The pristine preservation of intrusive relations at Oncócua Platform may mean that crystal transfer from magma to wallrock is more common in incrementally assembled granitoid plutons than previously thought.Supplementary material: [Mineral chemical data] available at https://doi.org/10.6084/m9.figshare.c.5448664

The Origins of Strongly Peraluminous Granitoid Rocks

Strongly peraluminous granites (SPAGs), with 1.20 < A/CNK < 1.30, are relatively rare rocks. They contain significant modal abundances of AFM minerals such as Bt-Ms-Crd-Grt-And-Toz-Tur-Spl-Crn of potentially magmatic, peritectic, restitic, and xenocrystic origin. Determining the origin of a SPAG depends to a large extent on establishing the correct origin of these AFM minerals. Strongly peraluminous granitic rocks can form in eight distinctly different ways: (1) as the melt fraction resulting from dehydration partial melting of peraluminous metasedimentary rocks; (2) as the bulk composition of diatexitic migmatite resulting from extensive partial melting of peraluminous metasedimentary rock; (3) as a diatexite modified by incomplete restite unmixing; (4) by bulk contamination of a less strongly peraluminous granite magma with highly peraluminous metasedimentary rocks; (5) by selective acquisition or concentration of AFM minerals by a less strongly peraluminous granite magma; (6) by fractional crystallization of quartz and feldspar from a less strongly peraluminous granite magma; (7) by removal of alkalies (Ca, Na, K) by release of a suprasolidus aqueous fluid from a less strongly peraluminous granite magma; and (8) by subsolidus hydrothermal alteration of a less strongly peraluminous granite rock. Contamination by pelitic material is the most effective process for creating SPAG plutons. A detailed case study of the South Mountain Batholith shows that its early SPAGs contain high modal abundances of Bt-Crd-Grt, largely of external origin, whereas its later SPAGs contain high modal abundances of Ms-And-Toz, largely the products of fluido-magmatic processes.

Bruce William Chappell 1936–2012

Bruce Chappell was one of the most distinguished geologists of his generation whose contributions to understanding the origins of granites are both insightful and profound. A pioneer in the application of X-ray fluorescence spectrography to the analysis of geological materials, his radical ideas about magma genesis, still the subject of vigorous debate, have dominated and largely determined the global directions of subsequent research on granites. His restite model, the recognition that most granite magmas move bodily away from their source regions as a mixture of melt and solid residual material, the progressive separation of which determines the magma composition, underlies his tenet that granites are images of their source. His consequent recognition, with Allan White, that there are two fundamentally different types of granite magma, I-type (derived from igneous sources) and S-type (derived from weathered sedimentary sources), each with its distinctive evolutionary path and associated mineralization, continues to underpin research into granites worldwide, and the search for granite-related mineral deposits.

THE GEOCHEMICAL CHARACTERISTIC OF MAJOR ELEMENT OF GRANITOID OF NATUNA, SINGKEP, BANGKA AND SIBOLGA

A study of geochemical characteristic of major elelemnt of granitoid in Western Indonesia Region was carried out at Natuna, Bangka, Singkep and Sibolga. The SiO2 contents of the granites are 71.16 to 73.02 wt%, 71.77 to 75.56wt% and 71.16 to 73.02wt% at Natuna, Bangka, and Singkep respectively, which are classified as acid magma. While in Sibolga the SiO2 content from 60.27 to 71.44wt%, which is classified as intermediate to acid magma. Based on Harker Diagram, the granites from Natuna, Bangka and Singkep as a co-genetic. In other hand the Sibolga Granite show as a scatter pattern. Granites of Natuna, Bangka and Singkep have the alkaline-total (Na2O + K2O) between 6.03 to 8.51 wt% which are classified as granite and alkali granite regime. K2O content ranges from 3.49 to 5.34 wt% and can be classified as calc-alkaline type. The content of alkaline-total of Sibolga granite between 8.12 to 11.81 wt% and classified as a regime of syenite and granite. The range of K2O is about 5.36 to 6.94wt%, and assumed derived from high-K magma to ultra-potassic types. Granites of Natuna, Bangka and Singkep derived from the plutonic rock types and calc-alkaline magma, while Sibolga granite magma derived from K-high to ultra-potassic as a granite of islands arc. Based on the chemical composition of granite in Western Indonesian Region can be divided into two groups, namely Sibolga granite group is representing the Sumatera Island influenced by tectonic arc system of Sumatera Island. Granites of Bangka and Singkep are representing a granite belt in Western Indonesian Region waters which is influenced by tectonic of back arc.Keywords: magma, geochemical characteristic, major element and Western Indonesian Region Kajian karakteristik geokimia dari unsur utama granitoid di Kawasan Barat Indonesia telah dilakukan di daerah Natuna, Bangka, Singkep dan Sibolga. Kandungan SiO2 granit Natuna antara 71,16 - 73,02%, Bangka antara 71,77 - 75,56%, Singkep antara 72,68 - 76,81% termasuk dalam magma asam. Granit Sibolga memiliki kandungan SiO2 antara 60,27 - 71,44% termasuk dalam magma menengah - asam. Berdasarkan Diagram Harker, granit Natuna, Bangka dan Singkep mempunyai asal kejadian yang sama (ko-genetik), sedangkan granit Sibolga membentuk pola pencar. Granit Natuna, Bangka dan Singkep mengandung total alkalin (K2O+Na2O) antara 6,03 - 8,51% termasuk dalam jenis rejim granit dan alkali granit. Berdasarkan kandungan K2O antara 3,49 - 5,34 %berat, bersifat kalk-alkali. Granit Sibolga mengandung total alkali antara 8,12 - 11,81% termasuk dalam rejim syenit dan granit, dan berdasarkan kandungan K2O antara 5,36 - 6,94% berasal dari jenis magma K-tinggi sampai ultra-potassik. Granit Natuna, Bangka dan Singkep berasal dari jenis batuan beku dalam dan magma kalk-alkalin yang berhubungan dengan penunjaman, sedangkan granit Sibolga berasal dari jenis magma K-tinggi - ultra-potassik sebagai granit busur kepulauan. Berdasarkan komposisi unsur kimia utama, granit di Kawasan Barat Indonesia dapat dibagi dalam dua, yaitu granit Sibolga yang mewakili P. Sumatera, dipengaruhi oleh sistem tektonik busur P. Sumatera. Granit Bangka dan Singkep dapat mewakili suatu jalur granit di perairan Kawasan Barat Indonesia yang dipengaruhi oleh tektonik busur belakang. Kata kunci: jenis magma, karakteristik geokimia, unsur utama, dan Kawasan Barat Indonesia