Garnet breakdown in some deep seated garnetiferous xenoliths from the central Sierra Nevada: Petrologic and tectonic implications

Abstract Garnets in garnetiferous xenoliths from Big Creek, Pick and Shovel and Chinese Peak volcanic pipes, occurring along the axial region of the Sierra Nevada batholith, have broken down to variable degrees to optically opaque and extremely fine-grained kelyphitic rims and seams. In some cases, only opaque pseudomorphs of garnet remain; in others, garnet occurs only as a tiny relict within an opaque mass; in still others, the kelyphite forms a rim and pervades the garnet grain in a complex reticulate fashion. Backscattered-electron imaging and electron microprobe analyses reveal that the unaltered portions of garnets exhibit negligible compositional variations. The unaltered portion is often surrounded by a resorbed rim whose composition is slightly different from that of the bulk garnet; however no systematic difference occurs between the compositions of resorbed rims and those of the unaltered garnets. Around the resorbed rim, lies a zone composed of symplectic intergrowths of aluminous spinel, aluminous orthopyroxene and calcic plagioclase (An91). Compositions of the symplectite spinels and orthopyroxenes are different from the compositions of those occurring as major discrete phases in the rocks. The possibility that the kelyphites results from oxidation is ruled out. Two alternatives for the origin of these kelyphites seem viable. In the first, they are the result of isochemical decomposition of garnet due to transport by the host lavas from depth to the surface. In the second, they are produced by breakdown and retrogression of garnet due to tectonic transport of the precursor rocks from high P-T conditions to lower P-T conditions before their incorporation into the host volcanics. The second alternative is favored and is supported by the following arguments: (1) Decompression due to transport of the xenoliths by the volcanics should essentially take place along an adiabat and would most likely cause partial melting. No melt phase was found in the symplectites. (2) Such rapid adiabatic decompression would arrest the original high P-T condition of equilibration of the primary xenolith mineralogy. The P-T conditions of equilibration of the primary xenolith mineral assemblages are found to range from the 11–24 kbar and 600–900°C. These low P-T conditions indeed indicate a re-equilibration of the primary mineral assemblages at a shallower depth. (3) The garnet breakdown reaction also involved clinopyroxene and produced an overgrowth of Fe- and Al-rich and Na-poor clinopyroxene around the primary clinopyroxene. (4) Using the known phase compositions, a balanced reaction, which involves primary phases and the symplectite phases, can be written. (5) Thermodynamic modelling of the garnet breakdown phenomenon according to the reaction: CaMgSi2O6 + 2Mg3Al2Si3O12=3Mg2Si2O6 + CaAl2Si2O8 + MgAl2O4 yields the P-T condition for garnet breakdown at 841°C-15 kbar for the sample PS-10, for example. The original subsolidus re-equilibration P-T condition of the primary mineral assemblages for this sample is calculated as 892°C-23 kbar. This calculation signifies uplift of the subsolidus garnet clinopyroxenite assemblage along a superadiabatic P-T path. The hypothesis set forth in turn suggests movement of the solid crust-mantle segments along a superadiabatic path from a high P-T regime to lower P-T conditions. Thus the crust-mantle segments underlying the Sierra Nevada batholith, represented by the xenoliths, originated at a greater depth and were tectonically uplifted prior to becoming entrained within the volcanic hosts. This dynamic model is compatible with the geological scenario of the western United States.