Petrology of the Bear Poplar and Barber mafic intrusions, North Carolina
The Bear Poplar and Barber mafic plutons are located in the Charlotte belt of North Carolina. The Bear Poplar pluton is a postmetamorphic intrusion consisting primarily of plagioclase, orthopyroxene, clinopyroxene, hornblende, biotite, and quartz. Magnetite, ilmenite, pyrite, pyrrhotite, and hematite are present in lesser amounts. The pluton is composed of four rock types that exhibit systematic areal trends within the intrusion. Cores of norite and gabbronorite are surrounded by hornblende norite and hornblende gabbronorite. Progressive differentiation trends characterized by Fe and Si0, enrichment exist in the pluton. Rocks of the Bear Poplar pluton are rich in both components compared to the Barber pluton. Textures are generally hypidiomorphic granular to inequigranular with some cumulates present.
The Barber pluton is a pre-metamorphic intrusion composed of three rock types that reflect the increasing metamorphic alteration of the pluton. Relict olivine-hornblende gabbronorites show the least amount of alteration and consist of plagioclase, orthopyroxene, clinopyroxene, hornblende, and olivine. Type I metagabbros contain some relict igneous features and are composed mainly of plagioclase, hornblende, orthopyroxene, and clinopyroxene. Type II metagabbros are the most altered samples and contain primarily plagioclase, hornblende, and epidote. Magnetite, ilmenite, pyrite, pyrrhotite, and hematite are present in lesser amounts. Textures range from relict hypidiomorphic granular to granoblastic.
Samples from both intrustions are generally medium-to-coarse grained, but finer-grained gabbroic rocks crosscut both plutons. These rocks are of similar mineralogy to the gabbroic rocks they intrude and denote multiple periods of intrusive activity for each unit. Unusual glomeroporphyritic samples are associated with the border of the Bear Poplar pluton and may reflect an early chilled phase of the magma.The Bear Poplar pluton crystallized from an aluminous magma of basaltic andesite composition, contrasted to the high-alumina to tholeiitic basalt parental magma of the Barber pluton. Increased fractionation of a basaltic magma could produce a basaltic andesite magma, similar to the Bear Poplar samples. Such magmas may be formed in an island arc or continental margin setting. Using major element discrimination diagrams, the composition of the Bear Polar pluton can be distinguished as a calc-alkaline basalt and that of the Barber pluton can be classified as a less differentiated island arc tholeiite. The calc-alkaline basalt is typically associated with intrusions within continental crust at converging plate boundaries and lends support to the idea that this complex is related to tectonic activity at a continental margin.
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