48. Transition from magmatic to K-metasomatic processes in granodiorites and Pyramid Peak granite, Fallen Leaf Lake 15-Minute Quadrangle, California
Lorence G. Collins
November 24, 2003
Alden Loomis studied the geology of the Fallen Leaf Lake 15-minute quadrangle in eastern California for his 1961 Ph.D. thesis. Many different granitic and relatively-mafic plutons (hornblende-biotite quartz diorite) occur in this area. These plutons exhibit such igneous features as contact metamorphic aureoles, mafic enclaves, schlieren, comb layering, dikes, hypidiomorphic granular textures, and strongly zoned plagioclase. Subsequent to the solidification of the plutons, deformation allowed hydrous K-bearing fluids to enter the solid rocks and cause K-metasomatism. In some places metasomatic K-feldspar cuts the borders of mafic enclaves and preferentially replaces their fine-grained plagioclase. Although Alden Loomis observed this K-metasomatism, he concluded that in comparison to the volume of rocks crystallized from magma, the amount of solid-state recrystallization and metasomatism was negligible. Because he reported myrmekite as an accessory, this study was initiated to see if the metasomatism was more extensive than he thought. Thin section analyses of textures show that as much as 28.5 volume percent K-feldspar was added to former deformed quartz diorite plutons, converting them into granodiorites while preserving igneous structures, hypidiomorphic granular textures, and remnants of zoned plagioclase crystals. Where deformation was relatively weak, only grain boundary seals were broken, and interstitial K-feldspar bordered by rim myrmekite replaced the exteriors of plagioclase crystals. Where greater deformation occurred, breaking not only grain boundary seals, but also microfracturing the plagioclase, the K-feldspar replaced both interiors and exteriors of the primary plagioclase crystals. In these places wartlike myrmekite was formed. In some places the metasomatic anhedral K-feldspar crystals grew to become nearly-euhedral K-feldspar poikiloblasts. Some parts of each granodiorite pluton still have remnants of a former quartz diorite, but some quartz diorite plutons still remain in the area unmodified. The relatively mafic Keith Dome quartz monzonite has not been modified, nor have parts of the Pyramid Peak granite where it exhibits diorite-hybrid-granite magma mingling and mixing. However, major parts of the Pyramid Peak granite have resulted from K-metasomatism of former quartz diorite which converted this rock into granite.
In his Ph.D. thesis, Alden Loomis (1961) mapped and described the geology in the Fallen Leaf Lake 15-minute quadrangle in eastern mid-California southwest of Lake Tahoe, and then later did additional mapping before publishing his studies as a quadrangle report (Loomis, 1983; Fig. 1).
In this report, he described metasomatism as resulting from late K-feldspar:
"In granitic rocks more mafic than quartz monzonite, nearly all of the K-feldspar is in late poikiloblasts which replace plagioclase preferentially, but all minerals to some extent. They are probably the last constituent to crystallize in most rocks. Many thin sections show late quartz which has developed some euhedral faces surrounded by K-feldspar.
The K-feldspar grains are easily visible in hand specimen because of the reflections from cleavage faces; they are not developed in any of the rocks to the extent of being individual crystals that stand out by themselves and that are not poikilitic. They are distributed very evenly throughout individual bodies on the scale of a cubic foot or so, but not on the scale of a thin section. The K-feldspar has replaced the ubiquitous mafic inclusions as easily as the host rocks, and commonly crystals are found across inclusion borders.
K-feldspar apparently did not precipitate as an early pyrogenetic mineral except in rocks more potassic than the normal granodiorites of the area. The alaskite at Rubicon Point, Pyramid Peak granite, Keiths Dome quartz monzonite, and Echo Lake and Phipps Pass granodiorites all have substantial amounts of early subhedral or euhedral K-feldspar. The dividing line between rocks that do and those that do not have early subhedral or euhedral K-feldspar is about 3.5 weight percent (about 2.5 mol. percent) K2O."
Alden Loomis' choice of 3.5 weight percent K2O as a dividing value for those plutons having or not having subhedral to euhedral K-feldspar is reasonable (1) because he reported that the Pyramid Peak granite contains 4.60 to 5.43 weight percent K2O and subhedral to euhedral perthite crystals and (2) because the Keiths Dome quartz monzonite contains 4.11 to 4.65 weight percent K2O and subhedral to euhedral perthite crystals. The Pyramid Peak granite shows no evidence of metasomatism where it crystallized with commingled diorite and hybrid rocks (Wiebe et al., 2002), and the Keiths Dome quartz monzonite also has not been affected by K-metasomatism. On the other hand, four granodiorite plutons (Camper Flat, Dicks Lake, Rockbound Valley, and Wrights Lake) contain 0.5 to 3.15 weight percent K2O (Loomis, 1983) and late, interstitial, anhedral, metasomatic K-feldspar. This relationship correlates with their being under the 3.5-weight-percent-K2O dividing value. Nevertheless, two granodiorite plutons (Echo Lake and Desolation Valley) contain 4.22 and 3.60 weight percent K2O, respectively (Loomis, 1983), and they seem to fit the 3.5-weight-percent-K2O dividing value because they have subhedral to euhedral K-feldspar in some places. But in other places these two granodiorites also contain late, interstitial, anhedral, metasomatic K-feldspar. On that basis, the added late K-feldspar could have increased the K2O content to more than 3.5 weight percent, and the 3.5-weight-percent-K2O dividing value is suspect. The presence of late metasomatic K-feldspar in these rocks raises the possibility that other granitic rocks in the area, such as the Phipps Pass granodiorite, which also has subhedral to euhedral K-feldspar crystals, may not strictly obey the 3.5-weight-percent-K2O dividing value either.
When Loomis suggested that: "The volume of granitic rocks formed by solid-state recrystallization and metasomatism is negligible," he likely was referring to those four granodiorites that had less than 3.5 weight percent K2O and made the logical assumption that subhedral to euhedral K-feldspar in granodiorites must mean that the K-feldspar had a primary magmatic origin. This assumption may not be the case. In some places the "late" anhedral interstitial K-feldspar could have grown to produce the larger poikilitic, subhedral to euhedral K-feldspar crystals, and such crystals would not have been strictly magmatic in origin (Collins, 1988). In that case, the value of 3.5 weight percent would represent the approximate dividing composition where granodiorites that contain anhedral K-feldspar formed by metasomatic processes grade into granodiorites that contain subhedral to euhedral K-feldspar that has also formed by metasomatic processes. Moreover, because much K2O is also contained in biotite, comparing weight percentages of K2O in the rocks may not be an accurate means of determining how the K-feldspar was formed in the rocks.
Because Alden Loomis listed myrmekite as an accessory in his petrographic descriptions of the granodiorites that contained late metasomatic K-feldspar, this study was initiated to determine what relationship the late K-feldspar had with the myrmekite and whether there was a larger degree of metasomatism than what Loomis had indicated. The results of this study were obtained on the basis of analyses of 336 thin sections of granitic rocks in the Loomis collection, 25 thin sections of samples that I collected in the Glen Alpine, Echo Lake, and Bryan Meadow granodiorites, and one thin section of a sample of the Keiths Dome quartz monzonite and 6 thin sections of samples of the Desolation Valley granodiorite adjacent to the northeast end of Aloha Lake (Fig. 1; samples collected by Forrest Hopson).
Many different kinds of plutonic granitic rocks occur in the Fallen Leaf Lake area (Fig. 1: one granite (Wiebe et al., 2002), one alaskite, ten granodiorites, one quartz monzonite, one quartz diorite, three diorite-gabbros, and one noritic anorthosite. Typical hand specimens of the Desolation Valley granodiorite, Camper Flat granodiorite, Keiths Dome mafic quartz monzonite, and Pyramid Peak granite are shown in Fig. 2. Most of the granitic rocks have hypidiomorphic granular or hypidiomorphic seriate textures and contain plagioclase crystals that exhibit strong compositional zoning with relatively calcic cores and more sodic rims. The strong zoning indicates relatively rapid cooling of magma in a shallow environment, which is supported by the coexistence of the plutons with a roof pendant of metavolcanic and metasedimentary rocks (Fig. 1). Mafic enclaves (Fig. 3, schlieren (Fig. 4), and comb layering (Fig. 5) are found in many of the granodiorite plutons. High-temperature metamorphic aureoles in the adjacent metasedimentary and metavolcanic wall rocks, dikes, internal contacts between multiple intrusions within a pluton, and external cross-cutting sharp contacts with adjacent plutons and with stratigraphic layers in the roof pendant are other features associated with the granodiorite plutons which support their primary magmatic origins.
In the granodiorites, the K-feldspar ranges from 0.1 to 28.5 volume percent and averages 13.3 volume percent (Loomis, 1983). The K-feldspar occurs either interstitially as small anhedral grains or as larger, crystallographically-continuous, anhedral to subhedral, poikilitic crystals that enclose smaller plagioclase, quartz, hornblende, and biotite crystals. Both small and large K-feldspar crystals coexist with rim myrmekite or wartlike myrmekite. Biotite ranges from 5 to 15 volume percent (averaging about 10 volume percent), and hornblende ranges from trace amounts to 11 volume percent. Therefore, much of the potassium in the granodiorites is contained in biotite rather than chiefly in K-feldspar. Titanite, allanite, and magnetite are common accessory minerals but are not found in all the granodiorites (Loomis, 1983).
In some of the granodiorite plutons, many places lack K-feldspar and have the composition of a hornblende-biotite quartz diorite. Where K-feldspar is present in the quartz diorite, the rock becomes granodiorite. K-feldspar makes its first appearance interstitially as irregular tiny grains that replace adjacent plagioclase crystals along grain boundaries and as penetrations into fractured plagioclase crystals (Fig. 6). In these places some of the plagioclase grains are bordered by rim myrmekite with very tiny quartz vermicules (Fig. 6). These places are gradational to where the K-feldspar crystals locally surround smaller grains of quartz, plagioclase, biotite, and hornblende, giving the K-feldspar a poikilitic fabric. Commonly, some of the plagioclase inclusions are bordered by rim myrmekite (Fig. 7). Tiny island remnants of unreplaced plagioclase may be found in the K-feldspar, and these remnants are in parallel optical continuity with larger adjacent plagioclase grains (Fig. 8, Fig. 9, and Fig. 10). Some islands and plagioclase grains outside the K-feldspar have serrate edges against the K-feldspar or have veins of K-feldspar extending into microfractures in the plagioclase (Fig. 9, Fig. 11, Fig. 12, Fig. 13, Fig. 14, and Fig. 15). Generally, the strong compositional zoning of the plagioclase crystals is preserved, and the replacement of the plagioclase by the K-feldspar is from the outside inward. In a few places, however, the interior of a zoned plagioclase crystal is replaced by a tiny island of K-feldspar bordered by rim myrmekite (Fig. 16). In most granodiorites the contacts between the K-feldspar and the replaced plagioclase are locally bordered by rim myrmekite that is concentric in most places with the zonation. In some places, however, the K-feldspar and the rim myrmekite may transect both albite twinning and zonation in the quart-free plagioclase (Fig. 16).
In granodiorites that exhibit evidence of cataclasis that breaks more than just grain boundary seals, wartlike myrmekite that projects into the K-feldspar (microcline) is formed instead of rim myrmekite. The quartz vermicules in the wartlike myrmekite tend to be coarser than in the rim myrmekite. The wartlike myrmekite is not a separate entity from the adjacent plagioclase because the plagioclase of the myrmekite is optically continuous with the quartz-free plagioclase (Fig. 17 and Fig. 18).
In a given thin section of granodiorite containing relatively abundant K-feldspar, much of the K-feldspar lacks any evidence of having formed by K-metasomatism. Therefore, one must use inductive reasoning that this K-feldspar is an end-stage product of complete K-metasomatism that is shown by the gradational replacements in other parts of the same thin section. Evidence for such gradational replacements includes the following. (1) All places in the granodiorite plutons which have no K-feldspar in quartz diorite grade to places with only small amounts of interstitial K-feldspar. (2) Some plagioclase plagioclase crystals are incompletely replaced, leaving island remnants of the plagioclase in the K-feldspar (Fig. 18). (3) Microfracturing of the plagioclase and breakage of grain-boundary seals allow interiors of broken plagioclase crystals to be replaced by K-feldspar that is bordered by rim or wartlike myrmekite (Fig. 19 and Fig. 20). (4) Where incomplete replacements are irregular and extensive, as in Fig. 20, patch perthite may result (Fig. 21, Fig. 22, and Fig. 23). (5) K-feldspar locally may replace parts of a former Carlsbad-twinned plagioclase crystal (Fig. 24) or aggregates of plagioclase crystals (Fig. 25) or may nearly completely replace a former Carlsbad-twinned plagioclase crystal (Fig. 26). And (6), in rocks exhibiting strong K-feldspar replacement of plagioclase, the replacing K-feldspar extends in some places from the granodiorite into a mafic enclave (Fig. 27), cross-cutting the boundary. Where this occurs, the plagioclase of the mafic enclave is preferentially replaced (Fig. 28). Commonly, at the outer margin of the crystallographically-continuous K-feldspar crystal against the mafic enclave, the small, unreplaced, remnant plagioclase crystals in the K-feldspar near the enclave contact are locally bordered by rim myrmekite (Fig. 29). The quartz vermicules in the rim myrmekite are tiny and barely visible under high power magnification. Similar preferential replacements of plagioclase by K-feldspar occur in the adjacent granodiorite to produce large poikilitic K-feldspar crystals (Fig. 30), and their abundance is what changes the former quartz diorite lacking K-feldspar into granodiorite. On that basis of the above six relationships, the evidence strongly suggests that all K-feldspar in the granodiorites has resulted from K-metasomatism of plagioclase in former quartz diorites.
Furthermore, in rocks that show strong cataclasis, the hornblende crystals commonly have a quartz sieve texture, showing that much of former hornblende has been replaced by quartz. Quartz replacements of hornblende in sieve textures also occur in noritic anorthosite associated with the Dicks Lake granodiorite (Fig. 31). Of course, once a hornblende crystal is totally replaced by quartz, there is no evidence remaining to indicate that the hornblende was once there.
Discussions of thin sections of samples collected for specific plutons repeat many of the above observations and have been placed in an Appendix for the interested reader. Such plutons include the Glen Alpine, Bryan Meadow, Echo Lake, Desolation Valley, Dicks Lake, Camper Flat, and Wrights Lake granodiorite plutons and the Keiths Dome quartz monzonite. Of particular interest, however, is the Pyramid Peak granite.
Wiebe et al. (2002) have studied the Pyramid Peak granite, concentrating on the area centered at Island Lake in the eastern part of the middle of the north-south elongated pluton (Fig. 1). Multiple sequences of diorite, hybrids, and granite layers occur here. Magma mingling and mixing are clearly demonstrated by diorite magma that chilled against granite tops, granitic pipes penetrating overlying diorite, load casts, and quartz xenocrysts in the diorite. The granite tops in each of these sequences clearly crystallized from magma. However, a one-kilometer-thick granite unit on the western side of the pluton, two kilometers west of Island Lake and west of Twin Lakes, which extends south for six kilometers to the southern end of the pluton, and another one-kilometer thick granite unit in the northern and eastern part of the pluton, which extends south from Red Peak area for four kilometers, show evidence of both having been formed by K-metasomatism of former hornblende-biotite quartz diorite. This hypothesis is supported by the occurrence of hornblende-biotite quartz diorite in the eastern border of the granite layer in the northern part of the pluton that has not been affected by K-metasomatism.
In the Island Lake area, the interlayered diorite-hybrid-granite sequences dip about 65 degrees westward, but near Twin Lakes they dip about 25 degrees eastward (Wiebe et al., 2002). The oppositely dipping layers were interpreted to be limbs of a syncline. Structural data plotted by Loomis, however, show that foliation planes in the western part of the pluton, west of the axis of the syncline and west of Twin Lakes, are nearly vertical in the thick granite layer. Moreover, similar vertical or steep dips occur in the thick granite layer in the northern part of the pluton south of Red Peak. Strong deformation that breaks and/or bends albite twin lamellae of the plagioclase crystals occur in these two thick granite layers. Many of the microfractured plagioclase crystals have been replaced along their margins by K-feldspar veins (Fig. 12 and Fig. 15) or have been replaced in their interiors to form patchy perthite (Fig. 20, Fig. 21, Fig. 22, Fig. 23, and Fig. 26). Some of the metasomatic K-feldspar crystals have increased in size to become large, poikilitic, equant K-feldspar (1-2 cm long) bordered by wartlike myrmekite.
Six arguments exist that can be used to support the subsolidus K-metasomatic origin of the K-feldspar crystals in the granodiorites and in parts of the Pyramid Peak granite rather than forming the K-feldspar crystals solely by magmatic processes from a late-stage melt fraction.
1. An argument used by Alden Loomis (1961, 1983) was that in any given place in a granodiorite, while the total modal feldspar content remains nearly constant, the modal plagioclase decreases as modal K-feldspar increases.
2. The occurrence of an anhedral K-feldspar crystal cutting a mafic enclave border (Fig. 27, Fig. 28, and Fig. 29) rules out crystallization of the K-feldspar from residual magma (Loomis, 1983). The enclave would not have a localized suspension of its hornblende and biotite crystals in a late-stage melt, allowing K-feldspar to crystallize from this melt, because other contacts of the enclave with the granodiorite show no evidence of partial melting or assimilation. Moreover, such cross-cutting K-feldspar crystals cannot be xenocrysts that result from magma mixing because the sizes and types of the mineral inclusions in the K-feldspar match that of the host enclave, and the minerals of the enclave project into the borders of the K-feldspar.
3. Rim myrmekite or wartlike myrmekite does not occur on all plagioclase grains, but (a) the simultaneous appearance of rim myrmekite with the first appearance of the K-feldspar, (b) the association of rim and wartlike myrmekite (Fig. 19, Fig. 20, and Fig. 33) with places that exhibit cataclastic breakage or internal microfracturing of the plagioclase crystals, and (c) the lack of deformation of the K-feldspar where adjacent plagioclase crystals are deformed or broken indicate that much of the K-feldspar formed under subsolidus conditions where deformation of the solidified plutons occurred (Collins, 1988, 1997a, 1997b, 1997c).
4. Hornblende, exhibiting a quartz sieve texture, both in the granodiorites and in noritic anorthosite (Fig. 31), indicates subsolidus Si-replacement of the hornblende because late low-temperature quartz would not crystallize in the cores of the early high-temperature hornblende crystals.
5. Because the K-feldspar normally forms late from a granitic melt, it would not have formed early in the relatively calcic cores of the zoned plagioclase crystals (Fig. 16) during crystallization of the magma as an antiperthite intergrowth. If antiperthite were to form, it would be expected in all plagioclase crystals, and that is not the case.
6. The serrate contacts between the K-feldspar and plagioclase crystals (Fig. 8, Fig. 9, and Fig. 17), the veins of K-feldspar extending into adjacent plagioclase crystals (Fig. 11, Fig. 12, Fig. 13, and Fig. 33), and the remnant islands of plagioclase in parallel optical continuity (Fig. 8, Fig. 17, Fig. 18, and Fig. 25) are consistent with subsolidus K-replacements of former plagioclase crystals.
Loomis (1983) described equant poikilitic K-feldspar crystals as much as 120 mm long in some of the granodiorites. These crystals, however, could only be seen because of sunlight reflections off cleavage surfaces. Because equant poikilitic K-feldspar crystals in the Echo Lake granodiorite have a metasomatic origin (see Fig. 32, Fig. 33, and Fig. 34), these Echo Lake equant metasomatic crystals cast doubt on a primary magmatic origin of the equant crystals in other granodiorites in the Fallen Leak Lake quadrangle. The development of equant K-feldspar crystals having a metasomatic origin is not unique to the granodiorites in the Fallen Leak Lake quadrangle. In other granitic terranes, early interstitial metasomatic K-feldspar crystals that are bordered by rim or wartlike myrmekite grow to become equant megacrysts that are poikilitic; see discussion of illustrations in the Papoose Flat granodiorite (Collins and Collins, 2002b), the Twentynine Palms quartz monzonite (Collins, 1997d), and the Caribou Creek granodiorite (Collins, 1999). Therefore, it is reasonable to suggest that equant K-feldspar crystals in the granodiorites and granite in the Fallen Leaf Lake quadrangle were formed by K-metasomatic processes.
Although many granodiorites worldwide are entirely magmatic in origin, having primary K-feldspar (orthoclase) in their compositions, the lessons learned from the granodiorites and the Pyramid Peak granite in the Fallen Leaf Lake quadrangle is that some granodiorites and granites obtain their granitic composition because of the addition of metasomatic K-feldspar following the solidification of an earlier-emplaced more-mafic, magmatic pluton. Alden Loomis (1983) mapped foliation planes consisting of aligned and/or lineated crystals that were parallel to the contacts of the plutons with wall rocks in almost all of the granodiorites. Therefore, these parallel structures could represent planes of weakness which would allow deformation, following the solidification of the magma in each pluton. The comb layering (Fig. 5), consisting of alternating cumulate concentrations of plagioclase and ferromagnesian silicates, and schlieren (Fig. 4) must have provided additional zones of weakness. Subsequent forces acting on these planar features could break grain boundary seals and microfracture the plagioclase crystals. The degree of breakage would determine the degree of K-metasomatism -- whether small interstitial K-feldspar crystals coexisting with rim myrmekite were formed or whether anhedral or equant K-feldspar poikiloblasts coexisting with wartlike myrmekite were formed.
It is logical that the younger, biotite-rich, quartz diorite magmas were intruded into older more mafic rocks where these older rocks were deformed and fractured by tectonic forces. In this way the younger quartz diorite magmas could incorporate broken fragments of the older mafic rocks as abundant enclaves. It should be equally logical that the forces that caused the initial breakage, allowing intrusion of magma, could continue after the younger quartz diorite plutons have solidified. Because this deformation would occur at temperatures below melting conditions, cracking of grain boundary seals and microfracturing of crystals in the younger quartz diorites could allow K-bearing fluids to move through the rocks in a plutonic scale. In this way former, relatively-more-mafic magmatic rocks could be transformed into metasomatic granodiorites and granite while preserving most of the original igneous fabric and structures. Furthermore, it is logical that a deep source of additional K existed that would transform the hornblende-biotite quartz diorites into granodiorites because of the large amounts of biotite (averaging 10 volume percent) in the quartz diorites. Some of the added K to produce the K-feldspar could also come from the localized replacement of some of the biotite by quartz in the deformed quartz diorites.
Certainly, the field appearances of the granodiorites do not suggest that any K-metasomatism had taken place because none of the igneous structures and fabric is fully destroyed during the replacement processes. The massive outcrops of the granodiorites (i.e., the Desolation Valley granodiorite, Fig. 35), the mafic enclaves (Fig. 3 and Fig. 35), the hypidiomorphic granular textures (Fig. 2), and the zoned plagioclase crystals still remain (Fig. 6, Fig. 8, Fig. 16, and Fig. 27). Therefore, the field appearances of the rocks may totally conceal the metasomatism that is present, and geologists studying such rocks need to keep this in mind.
Alden Loomis was the first to recognize that K-metasomatism produced K-feldspar in the granodiorite plutons in the Fallen Leaf Lake quadrangle. He should be commended for this perceptive observation in the early 1960s when K-metasomatism on a plutonic scale was not a hypothesis that was in favor. At that time, however, he would not have been aware of the significance of rim and wartlike myrmekite (Collins, 1988; Collins and Collins, 2002a), and, therefore, it would have been logical for him to conclude: "The volume of granitic rocks formed by solid-state recrystallization and metasomatism is negligible." Nevertheless, the presence of myrmekite is a clue that the granodiorites and parts of the Pyramid Peak granite in the Fallen Leaf Lake quadrangle achieved their granitic compositions because of the metasomatic formation of K-feldspar in earlier-solidified, biotite-rich quartz diorite.
Dr. Lorence G. Collins Department of Geological Sciences California State University Northridge 18111 Nordhoff Street Northridge, California 91330-8266 FAX 818-677-2820