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Magma Mingling and Magma Shearing in Corsica

 

Roberto Weinberg, Monash University, Australia

Jörn Kruhl Technische Universität München, Munich, Germany

 

 

Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL: users.monash.edu.au/~weinberg.

 

I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.

 

DISCLAIMER. The material on this website has not undergone the scrutiny of Monash University and does not conform to its corporate web design. It is entirely based on a free-spritied, curiosity-driven research effort by the author, and therefore in no way expresses the official position of the University.

 

In this webpage, we describe structures related to the interaction between mafic and felsic magma in the Hercynian granites forming the Corsica Batholith, in Corsica, France. They depict a wide range of structures depending on the melt fraction of interacting magmas and magma straining. We compare two outcrops depicting the mingling of two magmas, one weakly and the other one strongly strained during mingling, and then compare these two with a third outcrop where mafic magma intruded latek into a nearly solidified granite.
Click on each of the following or scroll down:
a) low-strain mingling of melt-rich magmas at Ota village;
b) high-strain mingling of melt-rich magmas in Golf of Valinco;
c) brittle intrusion of late mafic magma in Golf of Valinco.

Each section shows structures evolving from most ductile to most brittle.

CLICK ON ANY PHOTO FOR HIGH RESOLUTION VERSION

 

Geological Map of the Corsica Batholith, by Chocherie et al.

 

a) Magma Mingling in a Low-Strain Zone: From Pillows to Cracks
(Renna, M.R., PhD Thesis, Universita de Pavia, 2004)

 

Porto Complex, Ota Village

 

The early Permian (~280 Ma) Porto Complex is a shallow-level late-tectonic complex of western Corsica consisting of the central gabbro-granite complex known as the Ota association (Renna, PhD Thesis, Universita de Pavia, 2004). These two magmatic rocks show mingling relationships including mafic pillows and granitic pipes cutting through mafic layers. Mingled pillows sometimes comprise more mafic pillows, forming composite pillows, and may contain quartz and plagioclase occelli, and acicular apatite and skeletal ilmenite all resulting from mingling. Hybrid granites contain hornblende and mantled feldspars (rapakivi textures).

 

Mafic layer intruded by sub-vertical granite pipes

Granite pipes and alteration in gabbro. Upper
tip of pipe on the right expands and ends
in pegmatites.		 Elliptical cross section of pipes. Elliptical cross section of pipe.

 

Ductile disaggregation of mafic pillows in granite

Mafic pillows settled towards the right, on top of each other creating asymmetry. Mafic pillows partly separated by late, evolved, coarse quartz-feldspar melt segregated out of the original granite. Mafic pillow broken into two angular pieces by late, more evolved, coarse quartz-feldspar melt.

 

Crack development in pillows

Coarse quartz-feldspar melt vein into a pillow, recording the early stages of pillow disaggregation. Trail of irregular en echelon cracks propagating from the tip of a granitic vein. Detail of previous.



Narrow, en echelon cracks in mafic pillow. Detail of irregular cracks. Ductile fractures formed by the coallescence of pores (from Bluhm and Morrisey, 1965, in Eichhubl and Ayding (2003).


Radial cracks filled with pegmatitic material around a larger pegmatite vein. Radial diffusion around mafic enclave in granite.

 

b) Magma Mingling in High-Strain Zone in the Absence of Solid-State Deformation.

Golf of Valinco, Abartello Beach

This section depicts magmatic layering within an E-W trending magmatic shear zone in the southern part of the Corsica Batholith. The most characteristic feature of mingling in high strain, in contrast with low strain zones, is the development of layering, very similar in nature to tectonic, solid-state, sheared gneisses. The main difference is the lack of solid-state deformation at grain scale.

Lineation, when observed, is close to verticaal and defined by elongated grains of K-felspar and biotite. On vertical exposures, most commonly vergence of magmatic thrusts is to the north, defined either by melt-filled planes overlying drag folds, or simply by truncation of underlying layering. Early magmatic foliation and layering is folded and transposed into the dominant E-W trending orientation.

 

 

Magmatic Flow

Coarse, quartz-K-feldspar granite cuts through and spalls off the country granodiorite. A mafic pillow disaggregated by flow of the surrounding felsic magma to form a narrow biotite-rich band. Folded felsic dykelet in a preserved pod of mafic magma.

 

 

Magmatic Shear Zones: Thrusting to North

Isoclinal, ptygmatic folds of felsic layer. Notice that the photograph depictsa curved surface, facilitating the view but adding a distortion of the structures. Felsic magma flow leading to folding and transposition, parallel to the pencil, of early-formed biotite-rich band. Ductile, poorly localized drag folding of magmatic banding indicating thrusting to the left (north). Note well developed banding before thrusting.

 

 

Magmatic Shear Zones

Diffuse magmatic thrust to the north (right). Localized conjugate pair of thrusts, north to left. Localized shear zone with thrusting to the north (left).

 

 

Magmatic Shear Zones
Fold and faults on two melt-filled planes leading to fold pop-up Late stage brittle pegmatite dykes. Scale 1.5 m across.

 

Photomicrographs: Magmatic Textures (by J. Kruhl)

Mafic and felsic banding in mingling zones. Mafic bands are comprised mostly of plagioclase, biotite and amphibole, and felsic bands of plagioclase, quartz +/- biotite. Long dimension is 1.5 cm. Tightly packed plagioclase laths, biotite and hornblende defining a magmatic foliation. Notice lack of significant solid-state deformation. Randomly oriented plagioclase grains enclosed by biotite.

 

c) Brittle Mafic Intrusion into a Nearly-Solidified Granite.


Portigliolo, Golfo Valinco, WSW of Propriano village

Medium-grained syntectonic granite with a well-developed magmatic foliation is intruded by late mafic dykelets through a network of brittle cracks. The earliest stage of mafic intrusion encountered a granite still capable of deforming in a ductile fashion. The dykes intrude in a variety of orientations, but preferentially along 130o to 160o indicating an external control. Dykes developed a magmatic foliation at an angle to dykes margins, generally at 110o to 120o.

 

 

Mafic intrusions disaggregated and rotated by magmatic flow in granite (early stages of mafic intrusion)

 

Irregular mafic dykeles intrude coarse granite, possibly still containing small melt fraction, allowing for the irregularities (photo at high angle to dykelets). Layered mafic dyke broken up and stretched within granite. The lack of solid-state deformation in granite (confirmed petrographically, J. Kruhl, pers. commun.) suggests granite flowed as a partial melt. Layered mafic block rotated and stretched forming a sigmoidal enclave within granite lacking solid-state deformation.

 

 

Fracturing and disaggregation of granite by intruding dykes

 
Mafic intrusion disaggregating an effectively solid granite. Note fragments of coarse granite in mafic dyke and how the dyke splits into three tips. Fracturing of granite and mafic intrusion. Sharp mafic dyke with internal magmatic foliation nearly parallel to the upper margin, truncating magmatic folilation in granite parallel to pen (photo by J. Kruhl).

 

 

Fracturing of granite by intruding dykes (late stages of mafic intrusion)

 

Outward magma flow into parasitic
dykelet (vertical in photograph) disturbs
magmatic banding on feeder dyke.