In:
Mineralogical Magazine, Mineralogical Society, Vol. 61, No. 407 ( 1997-08), p. 557-572
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.% TiO 2 , 1% Cr 2 O 3 and 0.2% BaO. The smallest groundmass biotites have normal pleochroism but compositions unlike any previously reported, with ∼2% Al 2 O 3 , ∼8% TiO 2 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 (T DM = 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.
Type of Medium:
Online Resource
ISSN:
0026-461X
,
1471-8022
DOI:
10.1180/minmag.1997.061.407.08
Language:
English
Publisher:
Mineralogical Society
Publication Date:
1997
detail.hit.zdb_id:
2034522-7
SSG:
13
