Carbonatite dikes of the Chupadera Mountains, Socorro County

youngest pre-Tertiary intrusive rock and is
Carbonatites
not metamorphosed.
The carbonatites occur as a northeasterly
A thin wedge of Paleozoic limestone and trending, arcuate swarm of steeply dippin!
arkose unconformably overlies the southern dikes that cut the foliation of the Precam-
margin of the Precambrian rocks. These sedbrian rocks. The dikes typically are exposed
imentary rocks contain no carbonatite dikes within a thin, laterally extensive colluvium
and are presumed to be younger. Poor ex- as rows of boulders. Wall-rock contacts are
posures and major jasperoid zones make rarely observed (Fig. 3). The intrusives are
identification of the Paleozoic formations un- in long, narrow zones which, in some excertain.
However, Kottlowski (1950) and Egposures, contain up to three closely spaced,
gleston (1982) have documented the CaloJo adjacent dikes. Individual dikes commonly
(Mississippian) and Sandia (Pennsylvanian) ar6 0.2-q.9 ft thick. Select dikes have been
Formations in the map area.
traced continuously in outcrop for up to 200
To the north and east the Precambrian rocks ft along strike. Four dike zones exceed 1,000
are covered by a thick Cenozoic section of ft in shike length and may exceed 5,000 ft
ash-flow tuff, felsic volcanic flows, basalt, and with only minor lateral offset by faults. The
sedimentary rocks, which compose most of extent of carbonatites concealed by Paleozoic
the Chupadera Mountains (Eggleston, 1982). and younger rocks is unknown.
To the west the Precambrian rocks are down- Carbonatite textures range from fine to
thrown by the Chupadera fault.
coarse grained and from equigranular to por-
ExPlo
I
Alluvium I
cenozoic 5onlo Fe .9
broup ,:
terlrory votcontc ;
Josperoid
Poleozoic Iimestone I L
Corbonolite dikes
Scole
o 05
o I,OOO 2,OOO f t
FlgUlE 2-Gene:alized geologicmap_of carbonatites in the Chupadera Mountains (precambrian rocks
atter Kent, 1982; Tertiary rocks after Eggleston, I98Z; and mapping by the authors of this paper).
May 1986 Nm Mexico Geology
f
Gronile
Porphyritic schist
Amphibolite ond schist
Ouorlzo{eldspothic
schisl ond gneiss
lVico schist
\
o
"on,o.,
: side; dotted
if buried
strike ond dip
foliotion
phyritic. Many of the intrusives are microbreccias,
with rounded and irregular
carbonatite fragments set in a carbonatite
matrix with similar composition. Phenocrysts,
carbonatite fragments, and wall-rock
inclusions generally align to form flow banding
and local flowage segregation. Many dikes
are uniform in texfure, but some have variable
texture along strike. Wider portions of
others possibly are composite intrusives with
sharp contacts between different phases that
are distinguished by textural contrast.
The carbonatites are moderate brown on
fresh surfaces and yellowish brown to reddish
brown where weathered. Xenoliths and
phenocrysts of magnetite, apatite, and bio-
FIGURE 3-Outcrop of a 2.4-ft-thick carbonatite
dike. Wall-rock coniacts are concealed by colluvlum.
New AAex[c@
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