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TheConcord Gabbro-Syenite Complex is aPaleozoicpluton located inNorth Carolina.
The complex is mainly composed ofgabbro andsyenite and is of theSilurian-Devonian age.[1] The Concord Gabbro-Syenite Complex is a part of a larger suite of twentyplutons that form a chain extending almost 500 km, which lie within theCharlotteMetamorphic belt.[2] The complex is located inCabarrus County in south centralNorth Carolina. Many of the Concord plutons have minor quantities of syenite, which form horseshoe-shaped rings around the gabbro.[3] The Concord Gabbro-Syenite Complex within the Concord pluton is dated to 407 ± 36 Ma,[4] making it the oldest formation within the twenty plutons. It is thought unlikely that the plutons located within the Concord Suite formation spanned this much time, implying that complexes such as the Concord Gabbro-Syenite Complex most likely formed during an olderTaconicigneous suite.[5]
Magnetic modeling suggests that there are large quantities of magnetite concentrated within the gabbro and the syenite.[2][5] Despite this, there is no petrologic evidence that would confirm the existence of magnetite within the complex. Three possible explanations have been put forward to explain the magnetization of the two intrusive rocks:[4]
Early gravitational modeling showed that there was a deep gravity source located within the Concord Gabbro-Syenite Complex.[2] It was initially attributed to the depth of the gabbro and syenite plutons within the complex. Subsequent gravitational modeling assumed that the gabbro had a higher density (circa 280 kg/m3) than the surrounding country rock[5] but made no assumptions regarding the density of the syenite.[4]
The complex is made up of gabbro and syenite, but more specifically a olivine-plagioclase gabbro and clinopyroxene-plagioclase gabbro.[5]
The gabbro located within the Concord complex is made up of medium-grained olivine-plagioclase and clinopyroxene-plagioclase gabbro consisting of a variety of minerals such as:[4]
The syenite located within the Concord complex is made up of coarse-grained phenocrysts of perthite that are surrounded by other various minerals such as:[4]
| Oxide weight % | MnO | FeO1 | K2O | CaO | TiO2 | Na2O | MgO | Al2O3 | SiO2 | Total |
| Gabbro | 0.14 | 9.73 | 0.31 | 10.47 | 1.20 | 2.51 | 9.59 | 17.59 | 46.9 | 98.44 |
| Syenite | 00.19 | 3.95 | 4.41 | 2.26 | 1.05 | 7.45 | 1.27 | 17.10 | 60.8 | 98.48 |
It is thought that the meta-gabbros present throughout the Concord Gabbro-Syenite Complex were placed approximately 580-520 million years ago (inEdiacaran -Cambrian times) and that a subduction zone caused the igneous activity which created the meta-gabbros.[1] The crystallization of the gabbro and syenite is thought to have occurred 385-415 million years ago, placing itstectonic setting during the Silurian and Devonian periods. During this time, the Charlotte and Carolina slate belts were believed to be connected to the North American Continent against the central Piedmont suture. The thrusting of the belts into the western terrain could have resulted in crustal thickening, melting, and mantle uplifting.[1][5]