38. Modification of a magmatic tonalite to produce a megacrystal granodiorite by K-metasomatism, Monterey peninsula and northern Santa Lucia Mountains, California, USA
Lorence G. Collins
email: lorencec@cs.com
January 9, 2001
A megacrystal granodiorite, occurring in a pluton in the Monterey peninsula of California, is gradational south to quartz monzonite and then to tonalite and trondhjemite and southeast to granodiorite and then to tonalite. A primary magmatic origin for the pluton is indicated by normally zoned plagioclase crystals and by dikes that extend into biotite-hornblende quartz diorite southeast of Point Lobos and into biotite schists south of Carmel Valley Village. Plagioclase in the tonalite lacks zonation, but toward the northwest, the plagioclase becomes increasingly zoned in the megacrystal granodiorite. Therefore, the most shallow part of the original pluton must have been near Monterey. The tonalite is undeformed but progressively becomes cataclastically broken from the southeast toward the northwest. In less than one kilometer, broken plagioclase crystals are increasingly penetrated and gradually replaced by microcline, converting the tonalite into granodiorite. K-metasomatism first creates tiny microcline islands and veins in interiors of broken plagioclase crystals. Then, the microcline grows and engulfs plagioclase remnants and other groundmass minerals. Eventually, the microcline becomes megacrysts, some as large as 4 cm long. Wartlike myrmekite commonly borders the microcline. In formerly strongly-deformed zones, the microcline megacrysts locally have parallel alignment. Some megacrysts show concentric zonation and alignments of tiny plagioclase inclusions parallel to possible crystal faces of the microcline. Thus, the four facies in the pluton, tonalite, quartz monzonite, granodiorite, and megacrystal granodiorite, which seem to have formed by magmatic differentiation, are a result of cataclasis of a former trondhjemite/tonalite pluton in which some of the plagioclase has been replaced progressively by microcline.
A plutonic megacrystal granodiorite (Mgd), trending northwest-southeast from the Monterey peninsula, is gradational south to quartz monzonite (Qm) and unmapped trondhjemite and tonalite at and near Yankee Point, and southeast to granodiorite (Gd) and tonalite (T) south of Carmel Valley Village (CVV; see Fig. 1). The quartz monzonite extends as dikes (Fig. 2) into wall rocks of biotite-hornblende quartz diorite (Qd; Fig. 1), and the tonalite extends as dikes into large biotite schist enclaves (3 m wide; not shown on Fig. 1). The gradational boundaries between these granitic facies are approximately located. An additional large area of the megacrystal granodiorite occurs in the city of Monterey, but it was not studied and is not shown on Fig. 1; see geologic map of Dibblee (1999).
The megacrystal granodiorite (Mgd) has spectacular exposures on wave-cut rocks along the Pacific coast near Monterey, California (Fig. 3 and Fig. 4). In some places concentrations of megacrysts are local, but these are not separate pegmatite dikes (Fig. 5). Rare, but occasionally found, are concentrations of biotite in swirled schlieren (Fig. 6) in wave-cut rocks in the most western exposures of megacrystal granodiorite, southwest of Monterey in the west-facing coast, supporting a magmatic origin.
The megacrystal facies has been studied by a number of investigators (Bowen, 1965; Compton, 1966; Wiebe, 1970; Ross and Brabb, 1973; Dibblee, 1999). All of these investigators support magmatism as the sole origin of the megacrystal granodiorite and refer to it as being "porphyritic." However, Wiebe (1970) studied a smaller area of megacrystal granodiorite southeast of the area shown on Fig. 1 and suggested that "K-feldspar appears to have replaced plagioclase in granodiorite" and that a quartz monzonite facies exhibits metamorphic recrystallization.
The gradual changes from a more mafic to more felsic rocks --- (a) tonalite through granodiorite and then to megacrystal granodiorite or (b) tonalite and trondhjemite (unmapped) through quartz monzonite to megacrystal granodiorite (Fig. 1) --- strongly suggest that the different facies in this pluton have been formed by magmatic differentiation. However, a parallel alignment of microcline crystals in megacrystal granodiorite at Whaler's Cove and Bird Point in Point Lobos State Park (WC and BP, Fig. 1) in the western part of this pluton is unusual (Fig. 7) and attracted my attention because in other localities where megacrysts have been similarly aligned, myrmekite is common, indicating that some K-metasomatism has occurred. See Collins 1997b, 1997c, 1997f, 1998c). Therefore, this study was initiated (1) to investigate the extend of the parallel alignment of he microcline crystals, (2) to determine if the megacrysts were bordered by myrmekite, and (3) to find out whether magmatic differentiation is the proper interpretation for the origin of the different facies in the pluton.
>Fig. 1. Simplified geologic map (after Dibblee, 1999), showing location of the "porphyritic" (megacrystal) granodiorite of Monterey. Tonalite (T, green); granodiorite (Gd, orange), quartz monzonite (Qm, yellow); megacrystal granodiorite (Mgd, blue); quartz diorite (Qd, pink). Symbols: PP = Pescadero Point; WC = Whaler's Cove; PL = Point Lobos; YP = Yankee Point; BP = Bird Point; CVV = Carmel Valley Village. Tertiary and Quaternary sedimentary rocks have been omitted.
Fig. 2. Dike of quartz monzonite (light colored) extending into biotite-hornblende quartz diorite.
Fig. 3. Massive outcrops of megacrystal granodiorite along coast line of the Monterey peninsula. Larger crystals of microcline megacrysts (white; near bottom of photo) show alignment generally parallel to the coast line.
Fig. 4. Close-up of partly aligned megacrysts of microcline crystals. The largest crystals are as much as 4 cm long.
Fig. 5. Megacrystal granodiorite with local concentrations of megacrysts at Carmel Point, southwest of the city of Carmel (A HREF="lc38-1.gif">Fig. 1).
Fig. 6. Biotite-rich schlieren in west-facing, wave-cut exposures along the Pacific Ocean in the most western parts of the pluton, southwest of Monterey (A HREF="lc38-1.gif">Fig. 1).
Fig. 7. Parallel alignment of microcline megacrysts in granodiorite at Whaler's Cove, Point Lobos State Park, California (WC on Fig. 1).
The tonalite (T) south of Carmel Valley Village (CVV, Fig. 1) is biotite-rich (7-15%), undeformed, and lacks any K-feldspar. The amount of K-feldspar in the pluton generally increases to the northwest as the tonalite grades to granodiorite and then to megacrystal granodiorite. The plagioclase in the tonalite is unzoned, but toward the northwest, zoning appears and becomes increasingly apparent (Fig. 8), suggesting (1) that the top of the pluton was formerly centered around Monterey and (2) that its deeper root was south of Carmel Valley Village. Where megacrysts first appear in the transition to the megacrystal granodiorite, the megacrysts have a random orientation. This applies to both transitions from quartz monzonite to the megacrystal granodiorite near Yankee Point and from the granodiorite to the megacrystal granodiorite south of Carmel Valley Village (Fig. 1). Only in a central band about 2 km wide at Point Lobos are there a few local zones (30 m wide) containing a strong parallel orientation of the megacrysts, and these zones generally trend northwest-southeast (Fig. 7).
Fig. 8. Zoned plagioclase crystal; sericitized in core. Quartz (white); biotite (brown).
Gradual mineralogical and textural changes occur across the transition zones. These changes are accompanied by cataclasis, which can be seen only in thin section. Generally, there is no evidence in the field for cataclasis or deformation.
Tonalite. In the southeastern part of the pluton the biotite-rich tonalite (T, Fig. 1) is a medium-grained, hypidiomorphic, dark-gray rock and contains about 7-15 % biotite, 60-75 % plagioclase (An25-32), and 15-25 % quartz. Generally, the tonalite is massive and undeformed (Fig. 9). Gradually, northwestward, the tonalite becomes increasingly broken by cataclasis (a cracking of plagioclase grains), and microcline begins to replace the interiors of broken plagioclase crystals in irregular veins and islands in random distribution (Fig. 10, Fig. 11, and Fig. 12).
Fig. 9. Outcrop showing biotite-rich tonalite.
Fig. 10. Branched veins of microcline (light gray and white) extending into albite-twinned plagioclase (black).
Fig. 11. Veins of microcline (black) extending along curved fractures and replacing a large crystal of albite-twinned plagioclase (light gray).
Fig. 12. Irregular branched veins of microcline (white and light gray) in albite-twinned plagioclase (black) and also extended into an adjacent plagioclase crystal (light gray; right side) along fractures. Quartz (white); biotite (brown).
Progressively northwest (but not uniformly everywhere), the microcline islands coalesce as the microcline increases in abundance. Here, the evidence for the cataclasis is eliminated in many places because the coalesced microcline is unbroken. As the coalesced microcline areas (as seen in thin section) increase in grain size, tiny islands of plagioclase occur locally and are in optical parallel orientation with themselves and/or with a larger plagioclase inclusion inside the microcline (Fig. 13).
Farther northwest, where the microcline in veins and islands have coalesced and become larger crystals, myrmekite locally occurs with tiny quartz vermicules. The plagioclase of the myrmekite projects into the microcline but is optically with a larger plagioclase grain outside the microcline (Fig. 14). Where myrmekite first appears in the transition, its volume is equal to or exceeds the volume of the coexisting microcline.
Fig. 13. Remnant islands of plagioclase (white; lower right) in parallel optical continuity occur in microcline (gray) below an altered, large, remnant island of albite-twinned plagioclase with rounded scalloped edges. Biotite (brown).
Fig. 14. Myrmekite (center, top and bottom) on borders of albite-twinned plagioclase (whitish gray and gray) and projecting into microcline (black). Microcline penetrates and replaces plagioclase along cracks.
Granodiorite. Still farther northwest from the tonalite (Fig. 1), the microcline crystal become larger and extend beyond the boundaries of the initially replaced plagioclase grain into the surrounding, diversely-oriented, groundmass minerals. Here, the microcline encloses and/or replaces adjacent broken groundmass plagioclase grains and other minerals, preserving their diverse orientations (Fig. 15). Where this greater abundance of microcline is found, the rock is now a granodiorite (Gd, Fig. 1). In these places, however, it is difficult, if not impossible, to distinguish the microcline from the plagioclase in the field in an outcrop (Fig. 16).
Fig. 15. Microcline (gray) with partly replaced plagioclase inclusions. One albite-twinned plagioclase inclusion (center; right) has relatively straight edges parallel to the microcline lattice. Two other plagioclase inclusions (upper left, white; bottom right, black and light gray) have lattices that are inclined to the microcline lattice and have scalloped edges.
Fig. 16. Outcrop picture of biotite granodiorite.
Megacrystal granodiorite. Still farther northwest toward Point Lobos and Monterey, the microcline crystals increase to 1-2 cm long, becoming visible in the field as separate randomly-oriented megacrysts in the rock (Fig. 17). The rock now becomes the megacrystal granodiorite (Mgd, Fig. 1). Continuing northwest (8-10 km), the microcline megacrysts gradually increase in size and become the largest (4 cm long) at Point Lobos and in the coastal area southwest of Monterey. At Point Lobos the larger megacrysts tend to be concentrated in broad planar zones trending northwest-southeast. The dominant minerals in the megacrystal granodiorite are microcline (5-20 %), biotite (3-5 %), quartz (10-20 %), and strongly zoned plagioclase (An24-28)(60-75 %). Magnetite, zircon, allanite, and titanite are accessories. Chlorite, epidote, and sericite occur as alterations.
In the field the microcline megacrysts appear euhedral with sharp borders (e.g., Fig. 4). In thin section, however, these crystals commonly have ragged irregular boundaries. Margins of the microcline megacrysts project into deformed plagioclase crystals, which are commonly bordered by myrmekite with tiny quartz vermicules (Fig. 18 and Fig. 19). Island remnants of the plagioclase occur in the microcline in parallel optical alignment with themselves or with an adjacent large plagioclase grain outside the microcline (Fig. 18 and Fig. 19). Some of the large microcline megacrysts with straight instead of ragged edges have remnants of plagioclase inclusions (Fig. 20) with veins of microcline extending into fractures in these inclusions.
Fig. 17. Randomly oriented microcline megacrysts in megacrystal granodiorite.
Fig. 18. Carlsbad-twinned and zoned plagioclase crystal with speckled, sericite-altered core. Remnant end of the plagioclase crystal (center, right) occurs as an island in microcline (gray; right side) and is in optical parallel alignment with larger plagioclase crystal (left side). Myrmekite with tiny quartz vermicules occurs on corners of plagioclase projecting into the microcline.
Fig. 19. Albite-twinned plagioclase (dark gray) is bordered by myrmekite and projects into a microcline megacryst (grayish white). Plagioclase of the myrmekite is optically continuous with the larger plagioclase crystal. Optically parallel islands of the plagioclase (gray; right side, center) are enclosed in the microcline (grayish white). Biotite (brown). Sericite alteration (blue).
Fig. 20. Microcline megacryst (black) enclosing and projecting into a plagioclase island remnant (light tannish gray) along curved veins (former fractures; center). The veins have tiny microcline beads (center). One edge of the microcline forms a straight boundary (lower right). Biotite (brown).
In the transition from the granodiorite to the megacrystal granodiorite where the microcline crystals are about the same size as the adjacent groundmass plagioclase crystals, plagioclase inclusions are also found in the microcline. In these places the albite twinning and long edges of the these plagioclase inclusions are mostly aligned parallel to the microcline lattice (Fig. 21). When the albite twinning in a plagioclase inclusion is inclined to the microcline lattice, the inclusion has irregular scalloped edges instead of smooth borders (Fig. 22). In a few places the plagioclase inclusions form rectangular outlines, like that found in a zoned plagioclase crystal (Fig. 8). In one such place, a myrmekite fragment is also enclosed (Fig. 23).
Farther toward the northwest where the grain size of some of the microcline crystals gradually increases, the same relationships occur. Plagioclase inclusions have optical continuity with themselves or with larger plagioclase crystals outside the microcline. In the larger microcline megacrysts, however, the inclined plagioclase inclusions are greater in number than those which have parallel orientation. This would be expected because the plagioclase grains in the groundmass would have crystallized in random orientation in the magma, and, therefore, when these plagioclase crystals were engulfed by a growin microcline megacryst, rarely would they happen to have their lattices parallel to the lattice of the microcline. However, toward the northwest where the microcline megacrysts are as large as 4 cm long and have parallel alignment (Fig. 7), the plagioclase inclusions generally are smaller and more numerous, and those inclusions with parallel alignment are again more abundant than those with inclined lattices.
In many places, the edges of the large microcline megacrysts are bordered by aggregates of myrmekite grains that project into the microcline (Fig. 24). In these places the volume of the microcline megacryst is often more than 200 times greater than the volumes of the adjacent myrmekite grains, unlike what is observed at the first appearance of myrmekite.
Finally, in the Point Lobos and Monterey area, the megacrystal granodiorite is a rock which looks magmatic (Fig. 25). It has a uniform composition, contains zoned plagioclase, has euhedral microcline megacrysts that look like phenocrysts, shows no evidence for cataclasis or deformation, and some of the microcline has Carlsbad twinning. It should be emphasized that much of the primary zoned and unzoned plagioclase crystals still remains in the rock, and, therefore, much of the original hypidiomorphic texture is preserved.
Fig. 21. Aligned inclusions of plagioclase crystals (white) and biotite (brown) in zoned microcline megacryst (light gray) in optical parallel orientation.
Fig. 22. Aligned inclusions of albite twinned plagioclase crystals (bottom) in microcline megacryst (light gray; left to right) parallel to a possible crystal face in the microcline. Plagioclase inclusions (black and whitish gray) at the top have inclined lattices and exhibit scalloped edges.
Fig. 23. Possible remnant zoned plagioclase crystal light gray, with rectangular outline) enclosed in microcline (black). Myrmekite occurs on former border.
Fig. 24.. Aggregate grains of myrmekite projecting into borders of microcline megacrysts (dark gray; left and upper left corner) and white (on right side). Albite-twinned plagioclase (light gray, center). Biotite (brown). Quartz (white; upper right corner and bottom left edge of photo).
Fig. 25. Megacrystal granodiorite in which some microcline megacrysts are Carlsbad twinned (e.g., top left center).
Magmatism and K-metasomatism. In the field, the gradual changes from the relatively mafic tonalite to the more-felsic granitic facies, from bottom to top, in the Monterey pluton have most of the mineralogical and chemical characteristics typical of magmatic differentiation. Moreover, in a few places the Monterey pluton contains angular mafic enclaves that are fragments of the former solidified quartz diorite wall rocks. These enclaves were broken off and incorporated during intrusion of the magma that formed the pluton. Therefore, an initial magmatic origin of the original pluton is not in doubt. Nevertheless, a later K-metasomatism to produce microcline could have changed or modified the more mafic facies into the more granitic facies of the pluton.
It is obvious that the mafic enclaves in the pluton are wall rocks that were broken off by flowing magma when the wall rocks were totally solid and readily fractured. However, in the flowing magma carrying the enclaves, the early-formed plagioclase crystals floating in the magma or in a magma mush of crystals would not have been broken when the magma moved past the wall rocks because these crystals were surrounded by liquid (melt). In a liquid the pressure is equal in all directions, and cataclasis or shearing of floating crystals is not possible, nor would the enclaves be further sheared once they were enclosed in the liquid. Cracking of crystals occurs in solids free of liquid, just as cracking or fracturing of wall rocks to produce enclaves occurs in solids. Therefore, the tonalite of the Monterey pluton must have been totally solidified prior to deformation that caused the initial cracking of its plagioclase crystals.
Origin of microcline megacrysts and their plagioclase inclusions. On the basis that brittle deformation and cracking of crystals occurred in the tonalite of the Monterey pluton, avenues would have been created for K-bearing hydrous fluids to replace the plagioclase with microcline. These replacements would have occurred along the irregular fractures in the plagioclase, producing microcline in veins and islands (Fig. 10, Fig. 11, and Fig. 12). During gradual but incomplete replacement of the broken plagioclase, the coalescing microcline veins and islands from many centers of replacement would leave trapped tiny island remnants of plagioclase inside the microcline with parallel optical alignment with themselves and with other large plagioclase remenants (Fig. 13). Where a microcline crystal replaced and grew beyond the boundaries of the initial broken plagioclase crystal, it would enclose the diversely-oriented, groundmass plagioclase crystals (and other minerals). Therefore, many of the plagioclase inclusions in the growing megacrysts are remnants of many different groundmass plagioclase crystals and have random orientation. In some places the lattices of these remnant plagioclase inclusions are parallel with plagioclase crystals outside the microcline (Fig. 18, Fig. 19, and Fig. 20). Those enclosed, partly-replaced, plagioclase grains which happened to have their lattice orientations parallel to the microcline lattice are commonly replaced until their edges are nearly straight and parallel to the micocline lattice (Fig. 15, Fig. 21, and Fig. 22). In contrast, those enclosed, partly-replaced, plagioclase grains, which happened to have their lattice orientations inclined to the microcline lattice, are replaced with scalloped edges, and the microcline extends into them along fractures (Fig. 15, Fig. 21, and Fig. 22). These relationships must be common occurrences because both Schermerhorn (1956a) and Canon (1962, 1964) have also noted in other terranes that where microcline replaces plagioclase inclusions, those inclusions with inclined lattices have scalloped edges and those with parallel lattices have smooth, straight edges. Although orthoclase crystals growing in a magma could enclose earlier-formed plagioclase crystals as inclusions, the curved veins of microcline along former fractures in such inclusions (Fig. 20) are more characteristic of K-replacements of a broken, fully-solidified rock rather than forming the microcline by inversion from orthoclase crystallized from a magma in which fracturing of crystals would not have occurred.
Alignment of plagioclase inclusions and megacrysts. In the early stages of development of microcline megacrysts, some of the plagioclase inclusions are aligned parallel to the microcline lattice (Fig. 21 and Fig. 22). # Myrmekite is usually described as forming by exsolution from K-feldspar. However, if this early-formed myrmekite (Fig. 14) had resulted from exsolution, then its volume should be far less than the volume of the adjacent microcline. Instead, the volume of the myrmekite in this early transition stage commonly exceeds the volume of the adjacent microcline (Fig. 14). As determined experimentally, the amount of combined dissolved CaO and Na2O in a former high-temperature orthoclase crystal cannot exceed 16 % (Carmen and Tuttle, 1964), and, consequently, the maximum amount of combined CaO and Na2O in the adjacent volume of microcline or orthoclase which could exsolve to form the volume of myrmekite observed in Fig. 14 would be far less than what is seen. Therefore, the myrmekite cannot have formed by exsolution.
On the other hand, in Fig. 24, the volumes of the adjacent microcline megacrysts (extending beyond the field of view of the photo) are more than 200 times the volumes of the adjacent myrmekite grains. In that case, such myrmekite could exsolve from a former high-temperature orthoclase. Because the adjacent microcline megacrysts show no evidence of cataclasis, it would be reasonable to assume that these megacrysts are former primary orthoclase crystals from which exsolution occurred and, therefore, were former orthoclase phenocrysts. However, as is shown in the earliest stages of replacement, microcline is formed first in the interiors of broken plagioclase grains, and this microcline continues to grow and become the megacrysts. At no place is orthoclase observed. The myrmekite grains in Fig. 24 are broken, partly-replaced, plagioclase remnants and become clues to the former cataclasis that was eliminated when the microcline replaced most of the other parts of former broken plagioclase crystals. To understand the full origin of the microcline, one must study more than the single outcrop and observe what is found beyond the megacrystal granodiorite facies where the megacrysts disappear.
Inclusions in the microcline. If the large K-feldspar crystals in the various granitic facies of the Monterey pluton had grown as phenocrysts during late-stage crystallization from a magma and as a result of magmatic differentiation, then cataclastic fracturing should not occur in a liquid (magma), where the pressure is equal in all directions. Only in rocks that are totally solidified (below melting temperatures free of liquid) can breakage occur. On that basis, it is noteworthy that where a microcline crystal has replaced and grown beyond the boundaries of the initial broken plagioclase crystal and enclosed the diversely-oriented, groundmass plagioclase crystals, characteristic relationships are apparent. First, many of the inclusions are remnants of broken plagioclase fragments that are optically parallel with larger plagioclase remnants either inside the microcline (Fig. 13) or outside the microcline (Fig. 18, Fig. 19, and Fig. 20). Second, those enclosed, partly-replaced, plagioclase grains which happen to have their lattice orientations inclined to the microcline lattice are replaced with scalloped edges, and microcline extends into them along fractures (Fig. 15 and Fig. 23). These relationships are more characteristic of K-replacements of a broken, fully-solidified rock rather than crystallization of the K-feldspar crystals from a magma.
Zoning. The observed zoning of tiny, concentrically-oriented, plagioclase inclusions in the microcline megacrysts (Fig. 22 and Fig. 24) also have characteristic features. If the zoning in the megacrysts resulted from growth of former orthoclase phenocrysts in a magma, one would expect (1) that in a given thin section all the megacrysts would exhibit such zoning and (2) that both randomly oriented and parallel oriented megacrysts would exhibit the same degree of perfection of zoning, and neither of these situations is found. On that basis, the zoning in the microcline may reflect (a) an inheritance of zoning in a former zoned plagioclase crystal that is replaced by the microcline (Fig. 21) and/or (b) a condition where the microcline grows beyond the crystal border of an initial zoned or unzoned plagioclase crystal and encloses and replaces fragments of other adjacent plagioclase crystals. A strong cataclasis may produce many small plagioclase fragments and cause disorientation of these fragments. In that way the frequency in which many tiny grains happen to have their lattices parallel to the microcline lattice is increased. Those plagioclase fragments that have their lattices parallel to the microcline lattice tend to be unreplaced and occur in concentric rings, and those fragments that have their lattices inclined to the microcline lattices tend to be completely replaced or show ragged edges (Fig. 22 and Fig. 24). Such relationships are more consistent with K-metasomatism than with an origin of the zoned microcline megacrysts by magmatic crystallization.
Inherited magmatic characteristics. It is not surprising that all other geologists who have studied this megacrystal granodiorite (as typically seen in the Monterey area, Fig. 25), have considered it to be totally magmatic in origin and porphyritic. In this rock, most of the progressive steps leading to its final product have been eliminated by K-replacements and recrystallization. Moreover, everyone is well schooled that Carlsbad twinning must result during crystallization of orthoclase from a melt and that microcline can form only by inversion from orthoclase. So this knowledge can only reinforce a belief that the megacrystal granodiorite is totally magmatic. However, as in other localities Carlsbad-twinned microcline megacrysts can form by replacement of the interiors of former, cracked, Carlsbad-twinned plagioclase crystals without first forming orthoclase; see Collins (1997a, 1997c).
Quartz monzonite. In the area closer to the Pacific Ocean, mapped as quartz monzonite (Qm) near Yankee Point (YP, Fig. 1), similar transitional textural replacements in deformed rocks (like those in Figs. 5-24) occur from (unmapped) trondhjemite and tonalite (plagioclase An24-31) to quartz monzonite, and then to granodiorite with randomly-oriented megacrysts, and then to megacrystal granodiorite with strongly parallel-oriented megacrysts at Point Lobos (Fig. 7). Thus, in the quartz monzonite area (Fig. 1), deformation also controls the first appearance and formation of microcline megacrysts. Significantly, the quartz monzonite dike cutting the quartz diorite (Fig. 2) has been slightly deformed, and cracked plagioclase crystals have been replaced by microcline and myrmekite. These relationships are consistent with the hypothesis that megacrysts throughout the pluton are not localized sites of orthoclase phenocryst growth in magma but are microcline metacrysts formed by K-metasomatism.
Wall rock biotite-hornblende quartz diorite modifications. If large-scale K-metasomatism is a valid hypothesis, then it should be expected to cut across rock boundaries, and this is what is observed in the wall rock biotite-hornblende quartz diorite (Qd, Fig. 1 and Fig. 2), south of the quartz monzonite and megacrystal granodiorite of Monterey. This quartz quartz diorite is generally undeformed, but local cataclasis (cracking of grain boundary seals) has allowed some broken plagioclase grains to be replaced in their interiors by microcline islands. Here, also, small microcline crystals (1-2 mm wide) and myrmekite are formed in less than 1 percent of the rock. The volume of the associated myrmekite exceeds the volume of the adjacent microcline (as in Fig. 14). These relationships support the concept that K-metasomatism of biotite-rich plutons is a common phenomenon, following solidification, provided that deformation permits introduction of fluids to facilitate the breakdown of biotite to release the K.
Orthoclase versus microcline. Significantly, in all the different facies of the Monterey pluton, the only K-feldspar present is microcline. No trace of orthoclase exists. Microcline is the only K-feldspar in the veins or islands in broken plagioclase crystals in tonalite and the only K-feldspar in all the transitional stages to the megacrystal granodiorite. Dickson (2000, written communication), who has also studied the Monterey K-feldspar megacrysts, reports that on stained slabs the megacrysts have "hour glasses" of selected groundmass minerals (or incomplete concentric rings of groundmass plagioclase and biotite, as I have found) and oscillatory zoning.
If the K-feldspar megacrysts had formed initially as orthoclase that crystallized from a cooling magma, oscillatory zoning would be less likely. Instead, Ba ions would be enriched in the cores, because at high temperatures, Ba ions tend to crystallize before K ions and as the temperature drops, become less abundant toward the rims. Therefore, orthoclase crystals tend to be Ba-zoned from cores to rims rather than show oscillatory zoning. During K-metasomatism, however, the microcline megacrysts in the Monterey rocks would crystallize at nearly the same relatively-low temperature as the crystals grew from cores to rims. In that process, both Ba and K ions would be brought in continuously in fluids at this low temperature as they are subtracted from biotite being replaced by quartz. Because slight differences in Ba- and K-concentrations in these fluids would occur from time to time, oscillatory zoning is what should be expected.
Furthermore, the triclinicity of the microcline in the megacrysts can be easily explained because it replaces triclinic plagioclase (Wyart and Sabatier, 1956). Therefore, the microcline triclinicity in the Monterey pluton need not result from the secondary inversion from former orthorhombic symmetry in primary high-temperature orthoclase. For further discussion of this topic, see Collins (1998a, 1999a, 1999b) and Orville (1958, 1962).
There is no doubt that the trondhjemite and tonalite of the Monterey pluton had an earlier magmatic origin. The normally zoned plagioclase crystals must have crystallized from a rapidly cooling melt. The general absence of zoned plagioclase crystals in the tonalite in the southeastern area and the increasing degrees of zonation in plagioclase toward the northwest imply that what is now the megacrystal granodiorite in and around Monterey, crystallized near the former top of the pluton. The tonalite in the southeastern part must have been the facies that initially crystallized throughout most of the pluton, although the top may have been a trondhjemite. After final crystallization of the pluton, deformation of the early-formed facies permitted rising, K- and Si-bearing, hydrous fluids to move through and modify the trondhjemite and tonalite by K-and Si-metasomatism, converting them to quartz monzonite, granodiorite, and megacrystal granodiorite, depending upon the degree of cataclastic breakage and the available K and Si ions. In that process, much of the biotite was replaced by quartz, thereby releasing K and water to facilitate the K-metasomatism. A distant mantle source of the K ions is not necessary, although some K released from biotite in the pluton could have migrated upward to be concentrated at the top of the pluton. The K moved into cracked plagioclase crystals to form microcline, which locally grew in size, engulfing and replacing groundmass minerals to form megacrysts. Where deformation was strong and planar in zones of cataclasis, the microcline megacrysts were aligned parallel to the planes of shearing. The alignment likely developed because the long axes of deformed plagioclase crystals prior to replacement were rotated nearly parallel to the planes of cataclasis. The microcline megacrysts increase in size and abundance from the southeast to the northwest likely because the degree of cataclasis increased toward the northwest, and, thus, the system there became more open to fluid movements. The replacement origin of the microcline megacrysts means that the "porphyritic" granodiorite is not a true porphyry containing phenocrysts of orthoclase but the result of K-metasomatism. Nevertheless, the dikes that extended from the pluton and penetrated the wall rocks, the swirled schlieren characteristic of flowing plastic magma, and much of the hypidiomorphic texture are magmatic features that are retained during the replacement processes. Therefore, the magmatic tonalite and trondhjemite have changed or evolved into different mineralogical compositions after their emplacement and total solidification.
K-feldspar megacrysts lacking zoning and those with concentrically-oriented tiny plagioclase inclusions in rings have been studied by many different investigators. Suggested origins are either magmatic (Frasl, 1954; Cannon, 1964; Smithson, 1965; Hibbard, 1965; Booth, 1968; Emmermann, 1969; Smith, 1974a, 1974b; Vernon, 1986, 1989; and Smith and Brown, 1988) or metasomatic (Drescher-Kaden, 1948; Schermerhorn, 1956a, 1956b, 1961; Marmo, 1958; Stone and Austin, 1961; Augustithis, 1973; and Dickson, 1996). For those megacrysts considered by the above authors to be magmatic, the K-feldspar is either microcline or orthoclase. For those megacrysts considered to be metasomatic, the K-feldspar is usually microcline. Dickson (1996), however, considers the orthoclase megacrysts in the Papoose Flat pluton to be metasomatic; and Collins (1998b) finds that orthoclase bordered by myrmekite in the Cooma pluton in Australia is metasomatic.
Although progressive development of the microcline megacrysts in the Monterey granodiorite in cataclastically altered trondhjemite and tonalite supports a metasomatic origin for their formation, this situation does not necessarily serve as a model for other localities. Each terrane containing K-feldspar megacrysts with oriented, tiny, concentric plagioclase inclusions needs further study to see whether the characteristics are more consistent with K-metasomatism or magmatism.
Examples that are consistent with K-metasomatism include the megacrystal granite in the Waldoboro complex in Maine (Collins, 1997b), the megacrystal quartz monzonite at Twentynine Palms in the Joshua Tree National Park (Collins, 1997c, the megacrystal parts of the Ardara pluton in the Donegal granites (Collins, 1997d), the Ponaganset augen granitic gneiss in Rhode Island (Collins, 1997f), the megacrystal Kavala granodiorite in northern Greece (Collins, 1998c), the augen granite gneiss in the Bill Williams Mountains of Arizona (Collins, 1998d), and the megacrystal Bergell granite in the Swiss Alps (Wenk, 1982; Blanckenburg, et al., 1992; Schmid, et al., 1996).
Also, many granitic magmatic plutons in the Sierra Nevada have been modified by K-metasomatism. Moore (2000) reports that percentages of SiO2 and K2O increase from west to east across the central parts of the Sierra Nevada and suggests that this increase represents a transition from oceanic to continental dominated magma sources. Such progressive changes in magmatic sources are likely true, but the eastern, more-K-and Si-rich granitic rocks could have been further modified by K- and Si-metasomatism to help produce the higher percentages of SiO2 and K2O. Following solidification of the elongated magmatic plutons, plate tectonics has caused north-south, right-lateral shearing. Furthermore, plutons that are granitic are relatively buoyant, and even after solidification may continue to rise and cause vertical shearing. Cataclasis, resulting from this shearing, could open the rocks to vertical migration of K- and Si-bearing fluids that would produce K-feldspar (bordered by myrmekite) and additional quartz. The recrystallization and replacements, as in the Monterey megacrystal granodiorite, would eliminate most of the evidence for the cataclasis. Examples include modifications in a myrmekite-bearing albite granite in the Tungsten Hills quartz monzonite (Collins, 1988), the Rosy Finch shear zone in the Mono Creek granite (Tikoff and Teyssier, 1992), the megacrystal Mt. Whitney granite (Collins, 1988), and the Cathedral Peak granodiorite (Collins, 1988). Other similar nearby examples include K- and Si-metasomatically-modified, magmatic granitic plutons in the faulted and cataclastically sheared terranes of the Mojave Desert (Collins, 1997e).
I wish to thank Jerry Loomis, ranger at Point Lobos State Park, for arranging permission to collect samples in the state park. Mario Iglesias and Jerry Haas of the California American Water Company gave permission to collect samples along Sleepy Hollow Road north of the San Clemente Dam, and Don Lingenfelter drove me to various outcrops, which would not have been accessible without his help. Basil Tikoff loaned thin sections from the Mono Creek pluton. I thank my wife, Barbara, who did her usual fine job of making editorial suggestions that markedly improved the article.
Dr. Lorence G. Collins Department of Geological Sciences California State University Northridge 18111 Nordhoff Street Northridge, California 91330-8266 FAX 818-677-2820
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