HALOGEN BEHAVIOR IN ALKALINE IGNEOUS ROCKS: F AND CL IN APATITE AND BIOTITE IN BONE SPRINGS SILL, BIG BEND NATIONAL PARK, TEXAS

Halogen contents of apatites and micas have been studied in diverse igneous systems, but little work has been done on alkaline igneous rocks. I have collected and investigated samples of the internally differentiated analcime-bearing alkali gabbro of the Bone Springs sill from Big Bend National Park, Texas. Within the context of the Trans-Pecos Magmatic Province (TPMP), field relations place this intrusion into the period of early extension — i.e., post-compression, pre-Basin and Range faulting.

Bone Springs samples contain clinopyroxene + plagioclase + alkali feldspar + analcime + ilmenite ± olivine ± Ti-magnetite. Also, all rock samples contain fluoroapatite (X_F 0.4-0.8; X_Cl 0.02-0.03) and Ti-rich biotite (6.5 to >9.5 wt.% TiO₂) with 0.1-0.4 wt.% F.

Three map units have been delineated in the ~50 meter thick Bone Springs sill: the bottom half is the medium-grained lower border zone (LBZ) and the top ~5 meters is the aphanitic upper border zone (UBZ), both with the composition of alkali basalt on a TAS diagram. Between these is sandwiched the medium-grained to pegmatitic central border zone (CBZ), which is predominantly of alkali basaltic to phonotephritic composition, but with segregations (sheets, pods, ocelli, etc.) of light-colored rock equivalent to a tephriphonolite. Within the CBZ, these two end-member rocks may share sharp or gradational and intermingled contacts.

Apatite-biotite F-Cl-OH geothermometry (Zhu and Sverjensky, 1992) on gabbros in the CZ yield temperatures of 1000-1200º C, while those in the LBZ indicate 600-900º C. Interestingly, LBZ rocks that yield the lower temperatures contain the highest MgO and Ni values, and the lowest contents of alkalis and Zr. These data may call the thermometer into question for this system. The UBZ alkali basalt values for these elements fall between those of the CZ and LBZ. CZ gabbro biotites have higher Cl/F than those in the LBZ gabbro. Total halogen contents of biotite anticorrelate with biotite Mg#. The X_F of apatite and of biotite anticorrelate. The relationship between mineral halogen contents and stratigraphic height is complex, though apatite X_Cl is lowest at the top. Further work will investigate relationships between the crystallization/degassing history and the halogen behavior.