Oligocene Lamproite Containing an Al-Poor, Ti-Rich Biotite, Middle Park, Northwest Colorado, USA

R. N. Thompson, D. Velde, P. T. Leat, M. A. Morrison, J. G. Mitchell, A. P. Dickin and S. A. Gibson
Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK
Départment de Petrologie, Université P. et M. Curie (U.R.A. 736 du CNRS), 4, Place Jussieu, 75252 Paris, France
British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Department of Physics, The University, Newcastle upon Tyne, NE1 7RU, UK
Department of Geology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4MI
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

Abstract: A small 33 ± 0.8 Ma lamproite pluton is exposed in the midst of a 23–26 Ma basalt-rhyolite province in Middle Park, NW Colorado. It contains abundant phlogopite phenocrysts in a fine-grained groundmass of analcime pseudomorphs after leucite, biotite, potassic richterite, apatite, ilmenite and accessory diopside. The phlogopite phenocryst cores contain ∼4 wt.% TiO2, 1% Cr2O3 and 0.2% BaO. The smallest groundmass biotites have normal pleochroism but compositions unlike any previously reported, with ∼2% Al2O3, ∼8% TiO2 and F <1.5%. Apart from those elements affected by leucite alteration, both the elemental and isotopic composition of this lamproite are close to those of the Leucite Hills, Wyoming. Its Nd-isotopic model age (TDM = 1.6 Ga) is outside the Leucite Hills range but within that of other Tertiary strongly potassic magmatism in the region underlain by the Wyoming craton. Evidence from both teleseismic tomography and the mantle xenoliths within other western USA mafic ultrapotassic igneous suites shows that the total lithospheric thickness beneath NW Colorado was probably ∼150–200 km at 33 Ma, when the Middle Park lamproite was emplaced. This is an important constraint on tectonomagmatic models for the Cenozoic evolution of this northernmost part of the Rio Grande rift system.

Keywords: Oligocene lamproite • Al-poor Ti-rich biotite • richterite • lithospheric mantle

Mineralogical Magazine; August 1997 v. 61; no. 407; p. 557-572; DOI: 10.1180/minmag.1997.061.407.08
© 1997, The Mineralogical Society
Mineralogical Society (www.minersoc.org)