Extreme Fractional crystallization of a basaltic magma: The stillwater igneous complex

Stratigraphic, petrographic and geochemical evidence indicate that the volcano-sedimentary rocks of the Late Devonian Piskahegan Group, located in the northern Appalachians of southwestern New Brunswick, represent the eroded remnants of a large epicontinental caldera complex. This complex – the Mount Pleasant Caldera – is one of few recognizable pre-Cenozoic calderas and is divisible into Exocaldera, Intracaldera and Late Caldera-Fill sequences. The Intracaldera Sequence comprises four formations that crop out in a triangular-shaped area and includes: thick ash flow tuffs, thick sedimentary breccias that dip inward, and stocks of intermediate to felsic composition that intrude the volcanic pile or are localized along caldera-margin faults. The Exocaldera Sequence contains ash flow tuffs, mafic lavas, alluvial redbeds and porphyritic felsic lavas that comprise five formations. The Late Caldera-Fill Sequence contains rocks that are similar to those of the outflow facies and comprises two formations and two minor intrusive units. Geochemical and mineralogical data support the stratigraphic subdivision and indicate that the basaltic rocks are mantle-derived and have intraplate chemical affinities. The andesites were probably derived from basaltic magma by fractional crystallization and assimilation of crustal material. The various felsic units are related by episodes of fractional crystallization in a high-level, zoned magma chamber. Fractionation was repeatedly interrupted by eruption of material from the roof zone such that seven stages of caldera development have been identified. The genesis of the caldera is related to a period of lithospheric thinning that followed the Acadian Orogeny in the northern Appalachians.

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FRACTIONAL CRYSTALLIZATION OF BASALTIC MAGMA

Evolution of the Igneous Rocks: Fiftieth Anniversary Perspectives ◽

10.2307/j.ctt13x1dkm.9 ◽

2018 ◽

pp. 171-204

Author(s):

IKUO KUSHIRO

Keyword(s):

Fractional Crystallization ◽

Basaltic Magma

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The Atud gabbro–diorite complex: glimpse of the Cryogenian mixing, assimilation, storage and homogenization zone beneath the Eastern Desert of Egypt

Journal of the Geological Society ◽

10.1144/jgs2019-199 ◽

2020 ◽

Vol 177 (5) ◽

pp. 965-980

Author(s):

Robert J. Stern ◽

Kamal Ali ◽

Paul D. Asimow ◽

Mokhles K. Azer ◽

Matthew I. Leybourne ◽

...

Keyword(s):

Fractional Crystallization ◽

Mafic Magma ◽

Eastern Desert ◽

Content Type ◽

Igneous Complex ◽

Nubian Shield ◽

Partial Melts ◽

Supplementary Material ◽

Lower Crustal ◽

Arabian Nubian Shield

We analysed gabbroic and dioritic rocks from the Atud igneous complex in the Eastern Desert of Egypt to understand better the formation of juvenile continental crust of the Arabian–Nubian Shield. Our results show that the rocks are the same age (U–Pb zircon ages of 694.5 ± 2.1 Ma for two diorites and 695.3 ± 3.4 Ma for one gabbronorite). These are partial melts of the mantle and related fractionates (εNd690 = +4.2 to +7.3, 87Sr/86Sri = 0.70246–0.70268, zircon δ18O ∼ +5‰). Trace element patterns indicate that Atud magmas formed above a subduction zone as part of a large and long-lived (c. 60 myr) convergent margin. Atud complex igneous rocks belong to a larger metagabbro–epidiorite–diorite complex that formed as a deep crustal mush into which new pulses of mafic magma were periodically emplaced, incorporated and evolved. The petrological evolution can be explained by fractional crystallization of mafic magma plus variable plagioclase accumulation in a mid- to lower crustal MASH zone. The Atud igneous complex shows that mantle partial melting and fractional crystallization and plagioclase accumulation were important for Cryogenian crust formation in this part of the Arabian–Nubian Shield.Supplementary material: Analytical methods and data, calculated equilibrium mineral temperatures, results of petrogenetic modeling, and cathodluminesence images of zircons can be found at https://doi.org/10.6084/m9.figshare.c.4958822

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Field, geochemical, and isotopic evidence for magma mixing and assimilation and fractional crystallization processes in the Quottoon Igneous Complex, northwestern British Columbia and southeastern Alaska

Canadian Journal of Earth Sciences ◽

10.1139/e99-001 ◽

1999 ◽

Vol 36 (5) ◽

pp. 819-831 ◽

Cited By ~ 14

Author(s):

J B Thomas ◽

A K Sinha

Keyword(s):

British Columbia ◽

Fractional Crystallization ◽

Magma Mixing ◽

Parental Magma ◽

Geochemical Data ◽

Igneous Complex ◽

Source Regions ◽

Magmatic Body ◽

Color Indices ◽

Lower Crustal

The quartz dioritic Quottoon Igneous Complex (QIC) is a major Paleogene (65-56 Ma) magmatic body in northwestern British Columbia and southeastern Alaska that was emplaced along the Coast shear zone. The QIC contains two different igneous suites that provide information about source regions and magmatic processes. Heterogeneous suite I rocks (e.g., along Steamer Passage) have a pervasive solid-state fabric, abundant mafic enclaves and late-stage dikes, metasedimentary screens, and variable color indices (25-50). The homogeneous suite II rocks (e.g., along Quottoon Inlet) have a weak fabric developed in the magmatic state (aligned feldspars, melt-filled shears) and more uniform color indices (24-34) than in suite I. Suite I rocks have Sr concentrations <750 ppm, average LaN/YbN = 10.4, and initial 87Sr/86Sr ratios that range from 0.70513 to 0.70717. The suite II rocks have Sr concentrations >750 ppm, average LaN/YbN = 23, and initial 87Sr/86Sr ratios that range from 0.70617 to 0.70686. This study suggests that the parental QIC magma (initial 87Sr/86Sr approximately 0.706) can be derived by partial melting of an amphibolitic source reservoir at lower crustal conditions. Geochemical data (Rb, Sr, Ba, and LaN/YbN) and initial 87Sr/86Sr ratios preclude linkages between the two suites by fractional crystallization or assimilation and fractional crystallization processes. The suite I rocks are interpreted to be the result of magma mixing between the QIC parental magma and a mantle-derived magma. The suite II rocks are a result of assimilation and fractional crystallization processes.

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