Enclaves, Schlerien, Autoliths, Xenoliths And Dikes As Clues To

Structure And Pluton Exposure Level Of The Great Falls Metagranite On The Carolina Slate Belt- Charlotte

Belt BoundaryDonald R. Privett, P.G., Ph.D.

ENCLAVES, SCHLERIEN, AUTOLITHS, XENOLITHS AND DIKES AS CLUES TO STRUCTURE AND PLUTON EXPOSURE LEVEL - THE GREAT FALLS METAGRANITE ON THE CAROLINA SLATE BELT - CHARLOTTE BELT BOUNDARY, SOUTH CAROLINA

PRIVETT, Donald R.,1 Circle St, Great Falls, SC 29055, drprivett@gmail.com

Low-grade felsic and mafic rocks of the Carolina Slate Belt (?) and Charlotte Belt metadiorites and metagranites are intruded by the Great Falls Metagranite (GFM). The texture of the granite is mostly hypidiomorphic granular with minor porphyritic and aplitic textures. Quartz veinlets, minor small pegmatites and numerous narrow (0.2-1.5 m. wide) metamorphosed mafic dikes cut the metagranite.Larger rock exposures frequently display enclaves, mafic blebs, planar mafic layers, autoliths, xenoliths, aplite dikes and schlerien layers. Those features provide a record of the magma composition, flow and crystallization history of the older stoped and disrupted rocks.Medium-grained magmatic enclaves reveal complex histories and strain as the magma approached solidus. Enclaves are fine-grained relative to the granite and represent altered co-magmatic, injected material. Enclaves and schlerien are partly resorbed.

Mafic-rich blocks often have fairly rectangular shapes and a sharp, intrusive margin with little alteration, thus are xenoliths, granite sometimes cuts through a single xenolith. The metagranite stoped and assimilated or partially assimilated smaller mafic xenoliths, creating wispy ghostly almost indistinct autoliths.The pluton probably represents the deep-seated portion of an acidic sub-volcanic complex which was possibly a source for rhyolitic and mafic flows in the Carolina Slate Belt. Larger dikes cutting the granite may have been deep seated feeders for mafic flows of the slate belt.Some rock fragments in metagranite are unrelated to the igneous body itself. These xenoliths are located near their original positions of detachment or those with greater density (mafic) - deeper in the intrusion.In most exposures, metagranite is deeply eroded and altered to saprolite, outcrops are not sufficient to determine contact or reliable structural relationships.

Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Geologic Setting Map Carolinas

Green is Great Falls metagraniteYellow shows two near by 300 m.y.o. plutons

Mafic dikes cutting metagranite weathered to gravelly saprolite

in borrow pit..

Mafic dike cutting metagranite

Weathered vertical mafic dike cutting weathered metagranite.

Faulted nearly vertical mafic dike intrudded in to metagranite.

Magmatic enclaves are volumes of rock surrounded by emplaced host rock of related but distinct composition and of separated genesis (incomplete magmatic mixing). Enclaves are distinguished from xenoliths, which are fragments of metamorphically altered older country rock that fell into magma and became enveloped within igneous rock.ref:http://bio-geo-terms.blogspot.com/2006/06/schlieren.html

The origins of schlieren are not always clear; they may be produced by differential magma flow, or disaggregation of xenoliths, or by other mechanisms. Magmatic enclaves are volumes of rock surrounded by emplaced host rock of related but distinct composition and of separated genesis (incomplete magmatic mixing).

Enclaves are distinguished from xenoliths, which are fragments of altered older country rock that fell into magma and became enveloped within igneous rock.ref:http://bio-geoterms.blogspot.com/2006/06/schlieren.html

Schlieren are usually interpreted as having arisen by one of four mechanisms:1. shearing of heterogeneities (enclaves or xenoliths),2. crystal sorting during convective flow,3. crystal sorting during magmatic flow, or4. crystal settling.

http://bio-geo-terms.blogspot.com/2006/06/schlieren.html

Similar geology occurs along the North Carolina-South Carolina border near Waxhaw, NC, there regional low-grade metavolcanic volcaniclastic rocks of the Carolina terrane are intruded by the Waxhaw Granite a 539.4 ± 1.4 Ma crystallization age (U-Pb zircon, monazite, and xenotime) with greenschist-grade foliation. To the west, near the Charlotte terrane the Waxhaw Granite contains amphibolite xenoliths suggusting it is a stitching pluton.The Waxhaw Granite has been mapped in the footwall of the Gold Hill fault zone, a steep NW-dipping, dextral-reverse fault system.

Reference

ALLEN, John Stefan MILLER, Brent HIBBARD, James and BOLAND, Irene, 2007, SIGNIFICANCE OF INTRUSIVE ROCKS ALONG THE CHARLOTTE-CAROLINA TERRANE BOUNDARY: EVIDENCE FOR THE TIMING OF DEFORMATION IN THE GOLD HILL FAULT ZONE NEAR WAXHAW, NC, GSA, Southeastern Section

Foliated and sheared partly weathered metagranite.

Enclaves are also distinguished from schlieren, which are concentrations of mafic material that have

crystallized out of a single magma. Enclaves may represent the result of mingling of

mafic and felsic magmas.

Schlieren are fragile, usually elongate concentrations of mafic material. A schlieren could be a tabular zone in a granite with either more or less of some of the minerals in the surrounding granite, typically the dark (mafic) minerals.Schlieren may be produced by differential magma flow, or disaggregation of xenoliths, or by other mechanisms.

Partly re-sorbed xenolith.

Aplitic? enclave in metagranite.

Mafic xenolith cut by metagranite dike.

Close-up weathered aplitic metagranite.

Large metagranite exfoliation dome with narrow mafic schlerien (right of center).

Elongated mafic xenolith or narrow dike cutting weathered metagranite.

Mixed metagranite? with partly resorbed

mafic xenoliths or enclaves?

Altered mafic volcanic xenoliths, displaying epidote replacement.

WEATHERED METAGRANITE with small oval mafic ? schlieren

Mafic schlerien

Small partly resorbed mafic xenoliths

Partly resorbed mafic enclaves in metagranite.

Felsic enclave.

Water polished metagranite with flow layering of medium (upper) and fine grained layer.

Schlieren are usually interpreted as having arisen by one of four mechanisms:

1. shearing of heterogeneities (enclaves or xenoliths),2. crystal sorting during convective flow,

3. crystal sorting during magmatic flow, or4. crystal settling.

http://bio-geo-terms.blogspot.com/2006/06/schlieren.html

The origins of schlieren are not always clear; they may be produced by differential magma flow, or disaggregation of xenoliths, or by other mechanisms. Ref:http://bio-geo-terms.blogspot.com/2006/06/schlieren.html

? resorbed mafic