INTRODUCTION

Thousands of scientific papers about granite center around three main questions:

1) How is granite magma generated?
2) How does a large volume of granite magma transfer from its site of generation in the lower and middle crust and become emplaced in the shallow crust? and
3) how does a body of granite magma make room for itself when it intrudes the shallow crust?

Not one of the thousands of authors of those scientific papers knows the answer to any or all of the questions for sure, because not one of them was there when the plutons intruded. Those authors can only make inferences from equivocal field data, from time-limited laboratory experiments, and from questionable modeling studies. I view granite studies akin to those of a pathologist: From the corpse, s/he must infer its death, but from the granite, the geologist must infer its entire life history – its birth, its adolescence, and its death history.

Given that rather negative introduction, however, considerable insight really has been gained from the totality of field, laboratory, and modeling studies over the last 300 years. The study of granite has progressed greatly even since H. H. Read's 1947 pronouncement that "there are granites and granites".

This web site presents illustrations from studies of Birch Creek pluton, which offers a rather clear, but not entirely unequivocal, picture of its forcible emplacement mechanism.

Granite bodies are very large, commonly on the scale of tens to hundreds of cubic kilometers, large enough to require some unique process to replace, move, or remove the pre-existing rocks. The problem is one of transfer of mass – of a great volume of granite magma from depth into the shallow crust. The main ideas that authors over the years have proposed to explain how granite bodies make room for themselves are:

1) forcible emplacement: pre-existing rocks are shoved aside in a wholesale fashion by one large mass of granite magma (favored here for Birch Creek pluton);
2) dike intrusions: one thin dike after another intrudes and shoves aside piecemeal the pre-existing rocks (recently proposed by many plutons worldwide, but especially for some in the Sierra Nevada);
3) granitization or transformation of pre-existing rocks into granite by solid state processes (hardly anyone ascribes to this mechanism any more except locally on the small scale of tens or hundreds of centimeters);
4) assimilation or geochemical dissolution and transformation of pre-existing rock into granite by processes involving fluids.

Fairly convincing examples exist for the first two mechanisms (Papoose Flat pluton; McDoogle pluton), and the reader may spend an entertaining rainy afternoon searching the literature to find other examples. The emplacement mechanism for Birch Creek pluton is well described in the literature and should take only part of an afternoon to read.

Clem Nelson and I concluded that Birch Creek pluton represents a single batch of magma that intruded as a large dike along a fault on the flank of White Mountain anticline. The dike expanded with the influx of additional magma that forcibly shoved aside and overturned older rocks into the core of the anticline to make room for itself.

The pluton lies at the southern end of the White Mountains between Owens and Deep Spring valleys nearly at the latitude of Bishop, California. The pluton is well exposed so that the geologic relationships leading to the conclusions about forcible emplacement, stretching and thinning of wall rocks, are readily seen in accessible outcrops.

Most of the pluton can be reached only by shank's mare after a some rough driving up Molly Gibson Canyon, Birch Creek, or from the north via Wyman Canyon and thence down an unnamed jeep track. None of these roads comes closer than one half mile of the pluton. The pluton's relative inaccessibility explains why so few studies have been done of it and so little has been written about it

Birch Creek pluton is one of several granitic plutons in the White-Inyo Range of eastern California that are generally regarded as satellites of the great Sierra Nevada batholith, which lies 20 km to the west. The granitic compositions and Jurassic-Cretaceous ages of the White-Inyo plutons are closely similar to those in the Sierra. The fundamental difference is that the White-Inyo plutons are largely encased in sedimentary country rocks whose ages and former stratigraphic geometry are known or confidently inferred; only a few screens and pendants of former country rocks are found in the Sierra. There plutons intruded each other in most instances, so it is hard to know how any given pluton made room for itself. Thus it is possible to infer the damage the White-Inyo plutons did to the country rocks more confidently than for the Sierran plutons.

The country rocks in the White-Inyo Range, although extensively folded and faulted, are not metamorphosed except around plutons. Thus, the metamorphism and associated structures date from the intrusion of the plutons. This is a fundamentally important conclusion, because in so many other plutonic terranes, plutons intruded rocks that had already experienced multiple metamorphic events, so that only with great difficulty may one unravel the metamorphism and deformation associated solely with pluton emplacement. Not so at Birch Creek.

Birch Creek pluton is shaped like an inverted bell in plan view, being about 6 km-long in the north-south direction and a maximum of 5 km wide. It was emplaced in the east, nearly vertical limb of the White Mountain anticline. The granite itself consists largely of pale gray, biotite quartz monzonite that grades imperceptibly into biotite granodiorite close to the contact with the country rocks. The granite is remarkably homogeneous texturally, being coarse and even grained in the core and finer-grained and faintly foliated around its margin. Distinctive internal contacts are lacking. I have seen only two, 20 cm-long mafic enclaves in the entire pluton. It has a monazite age of 78 ma (Ayers et al., 2006).

Birch Creek pluton offers one of the clearest demonstrations of forcible emplacement of any of the plutons I have ever worked on or visited in field trips. That demonstration is clearly evident from the geologic map that shows the damage done to the internal structure of the White Mountain anticline by the pluton. The axis of the anticline trends and plunges south. It contains several subsidiary folds that also originally trended south, and the fold contains several steep-dipping N-S striking faults. The faults, folds, and fold axes were forcibly shoved west and northwestward by the pluton during its intrusion. Wyman and Reed formation strata were also overturned by the pluton as it shoved west, as can be seen clearly in the field from geopetal structures in the Reed Formation and by the attitudes of strata when followed around the north and west sides of the pluton from upright attitudes, to overturned attitudes, and then around a synformal anticline back to upright attitudes on the west flank of the main anticline.

We know the folds and faults existed before emplacement of the pluton, because the southernmost end of the pluton clearly cuts one of the faults.

Recrystallization is main contact metamorphic effect of the pluton's elevated temperature and pressure on its country rocks. Locally masses of massive epidote and brown garnet are found in a prospect pit at the northwest corner of the pluton, but neither of these minerals individually or together tell much about the pluton's emplacement conditions.

Also at the northwest corner granite dikes intrude the Reed Formation and have been deuterically altered to greisen, which is an assemblage of quartz, white mica, and fluorite. Small, pale blue beryl crystals have been reportedly found here, too, along with traces of copper minerals and gold.

Physical conditions of emplacement are commonly determined from mineral parageneses in rocks surrounding the pluton. In the case of Birch Creek pluton, however, such diagnostic mineral assemblages are lacking in the generally anydrous greywacke of the Wyman Formation and dolomite of the Reed Formation.

EMPLACMENT MECHANISM

Clem Nelson and I concluded that Birch Creek pluton exploited a north-northweast-striking fault as its initial locus of intrusion. East of the fault, emplacement appears to have been "passive", because the wall rock strata remain undeflected from their original geometry, whereas west of the fault, emplacement was clearly forcible at the present level of exposure. We suggested that decarbonation of the dolomitic shell around the pluton caused isothermal crystallization of the granite and prevented it from further intrusion except northwestward where, judging from out structural interpretations, the Reed Dolomite is attenuated or largely absent as a result of pre-intrusion faulting.

Birch Creek pluton, which is similar in lithology,age and emplacement mechanism to Papoose Flat pluton, clearly made room for itself by shoving its country rocks northwestward, whereas the eastern half of each pluton just as clearly has a more passive mode of emplacement. And just like Birch Creek, Papoose Flat pluton ascended to a carbonate and rose no higher in the stratigraphic succession, suggesting that a magma may crystallize isothermally upon decarbonation of its carbonate wallrocks and become immobilized.