Crustal xenoliths from Potrillo Maar: implications for evolution of continental crust beneath the southern Rio Grande rift

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Title: Crustal xenoliths from Potrillo Maar: implications for evolution of continental crust beneath the southern Rio Grande rift
Author: Chattopadhyay, Indrani
Abstract: The mid -crustal xenoliths from Potrillo maar record a petrologic and geochemical history that has until now been unknown from the Potrillo volcanic field . The igneous and meta -igneous xenoliths are samples of one or more plutonic bodies that crystallized at depths from 7 -14 km (middle to upper crust ) . The rock unit ranges in composition from hornblende biotite diorite to quartz monzonite and monzogranite ; it is broadly alkalicalcic The initial Sr87Sr86 suggests that the source region is heterogeneous or that the parental magmas were contaminated by crustal material . At least three geochemical groups can be identified , primarily on the basis of silica content . There is considerable overlap between groups 1 and 2 , and the boundary between these two groups cannot be distinguished graphically . The third group is granitic and is geochemically distinct from groups 1 and 2 . A dioritic composition is thought to represent mafic parental magma . Its calculated liquidus temperature at 2 % H2O and 4 Mpa pressure is 1172°C . The presence of fossiliferous limestone xenoliths indicates that the Potrillo maar eruption sampled rocks of late Paleozoic age . However , the age of marble and quartzite xenoliths are not known . The quartzite xenoliths are relatively undeformed , but isoclinal folding in some of the marble xenoliths suggests subsurface deformation that has not been observed in local surface exposures of metasedimentary rocks . One model proposed for the crust under the southern Rio Grande rift assumes that the marble is late Paleozoic in age . The fossiliferous limestones are late Paleozoic in age and are older than the plutonic rocks . The relative age of the deformed marbles in this model is not clear , it can be deformed with the plutonic rocks or due to an earlier regional extensional event . In the second model , marble is proposed to result from contact metamorphism in the aureole of a plutonic rock body at a depth of 7 -14 km . The presence of marble and calc -silicate xenoliths in dioritic blocks is consistent with a correlation of the marble with surface exposures of the Castner Marble in the Franklin Mountains
URI: http : / /hdl .handle .net /2346 /15611
Date: 2003-08

Citation

Crustal xenoliths from Potrillo Maar: implications for evolution of continental crust beneath the southern Rio Grande rift. Master's thesis, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /15611 .

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