EVIDENCE FOR A CRYSTAL-SETTLING MODEL FOR THE FORMATION OF LAYERED-PEGMATITE-APLITE INTRUSIVES (LPAI)

LPAI are characterized by horizontal*, tabular forms and range in thickness from approximately 1 to 20 m. They have two major layers--a top of cm-grained, pegmatite rock and a base of mm-grained, multilayered aplite. The core zone separates the two major layers and ranges in thickness from 0 to about 0.5 m. The pegmatite bodies in the Pala Pegmatite District, San Diego Co., CA are of this type. Such bodies are neither rare nor common and typically occur in groups.

A number of textures and features strongly suggest homogeneous nucleation plus an accumulation of grains (sedimentary process) for the formation of the multilayered aplite. Features of interest in this part are:

• drop-block xenoliths;
• draping of layers over xenoliths;
• graded bedding (uncommon);
• individual layers usually consist of garnet±tourmaline+quartz+feldspar;
• squeeze-ups;
• ‘comb-structure’ layers (irregularly present);
• great linear extent of individual aplite layers;
• euhedral garnet and tourmaline grains.

Features of interest in the total pegmatite body are:

• isolated, scattered, dm-grained, tear-drop-shaped K-feldspar in the upper part;
• non-uniform distribution of K-feldspar grains;
• anhedral grains of garnet and tourmaline in the upper part vs. euhedral grains in the lower, aplitic part;
• symmetrical compositional variation of the garnet across the body coupled with asymmetric abundance;
• symmetrical occurrence of tourmaline about the core zone coupled with a statistically horizontal attitude of this euhedral, elongate-habit mineral in the aplitic part;
• brown-clay and rubble-filled ‘former’ miarolitic cavities;
• asymmetric location of the core zone;
• consistent location of multilayered aplite in the lower and cm-grained rock in the upper part.

Other features are:

• consistent low-Si composition of adjoining, igneous country-rock;
• extensive alteration halos*;
• thin, horizontal*, tabular bodies of km length and breadth.

A complex part of the history of these bodies is the hypothesized variations in the water content which results in a number of ‘odd’ features.

Note that this model for the formation of LPAI bodies is in direct contrast to the diffusion models of several geologists as summarized in London (2008-Can. Min. Spec. Pap. 10).