In common with many other volcanoes on Java, rock compositions at Sumbing display a restricted range from basaltic andesite to andesite. All of the rocks have a standard mineral assemblage of plagioclase, pyroxene and titanomagnetite, and most are porphyritic containing megacrysts, phenocrysts, microphenocrysts and cumulophyric clots. Hydrous accessory phases include amphibole and biotite which appear to be inherited from cumulates and/or xenocrysts. The samples are described based upon groups of similar eruptive products, provenance, mineral assemblage and chemistry.
3.3.1. Pyroclastic Deposits (Sumb69, 71 & 94-99)
The pyroclastic deposits are summarised in figures 3.3 (a-c). These rocks all have a basaltic andesite composition and phenocryst phases are dominated by plagioclase, clinopyroxene and titanomagnetite. Three of the samples (including the two from Kwandungan) contain phenocrysts which are set in a brown glassy matrix with irregular vesicles containing glass inclusions. They are moderately porphyritic, with phenocryst
Pyroclastic Deposits
Fig. 3.3 (a) Field photograph showing the
pyroclastic deposits which are exposed on the lower flanks of Sumbing. The volcanic clasts range from meter-sized blocks to centimetre-sized fragments and are basaltic andesite in composition.
Fig. 3.3 (b) Photomicrograph of a pyro-
clastic basaltic andesite (Sumb95) showing the highly porphyritic nature of these rocks. Samples have high modal proportions of plagioclase and clino- pyroxene phenocrysts with textures indicative of melt disequilibrium such as inclusions, resorption and oscillatory zoning (in plagioclase minerals).
Fig. 3.3 (c) Plagioclase analyses of the
pyroclastic deposits (Sumb69 and 94) show a wide distribution in Anorthite (An) compositions. Insert displays core and rim mineral analyses for the plagioclase phenocrysts. An = Anorthite, Ab = Albite and Or = Orthoclase.
contents up to 50%, the largest of which are around 2 mm in length. The other samples (all from the southern flank) are more phenocryst-rich (up to 80%) and set in a fine-medium grained groundmass of plagioclase, clinopyroxene and titanomagnetite (fig. 3.3b). These rocks have higher proportions of clinopyroxene and titanomagnetite, which are often clustered together, or in cumulophyric clots with plagioclase. In addition to the higher proportion of crystals, phenocryst sizes tend to be larger in the latter, with many in excess of 1 mm.
3.3.1.1. Pyroclastic Phenocryst Phases
Plagioclase is by far the most dominant phenocryst phase in all samples. Crystals are commonly subhedral to euhedral laths, but can also be rounded or fragmented and display ubiquitous evidence of melt disequilibrium. Textures including normal, reverse or oscillatory zoning, concentric core-to-rim melt inclusions, resorption and partial replacement to clinopyroxene are prevalent, particularly in the more crystalline samples where these features tend to be more prominent.
The phenocrysts from this group display a large range in compositions (fig. 3.3c), from An 36 to An 92, with the highest values located in the cores. Within a single plagioclase phenocryst, anorthite content can range from An 60 to An 92 in the Old Sumbing Deposits and An 61 to An 90 in Kwandungan. This is not a characteristic observed in all feldspars, with many crystals exhibiting a more restricted range within the labradorite field. The most sodic plagioclase has an orthoclase content of ~10%, a value far higher than for any of the lavas. Clinopyroxene phenocrysts are ubiquitous in the pyroclastic deposits, and account for a significant proportion of the modal rock volume, particularly in the more crystalline rocks (up to ~10% of the total rock volume). Clinopyroxene is the most dominant ferromagnesian mineral occurring as phenocrysts, microphenocrysts, aggregates (commonly with titanomagnetite) and intergrowths (usually in other partially resorbed minerals). Euhedral to subhedral crystals range in size from less than 0.1 mm to over 1 mm, and are often twinned and less frequently zoned. All of the phenocrysts are classified as aguites (fig. 3.5), with a restricted range in compositions (En40-45, Fs14-19, Wo40-44). However, the more crystalline
samples contain slightly higher wollastonite content (Wo41.3-43.7) compared to the glassy
samples (Wo39.8-41.8), which correspond with a higher abundance of clinopyroxene in these
The minor mineral phases include titanomagnetite, orthopyroxene, apatite and amphibole. Amphibole is not evident in all samples and is often partially (or completely) resorbed and/or replaced by clinopyroxene. A single megacryst of amphibole in Sumb98 exceeds 0.5 cm and is rimmed by small plagioclase, orthopyroxene and Fe-Ti oxides which suggests that it was grown under different magmatic conditions and possibly represents an inherited mineral. Titanomagnetite and orthopyroxene are phases in all of the basaltic andesites, often occurring as clots of ferromagnesian minerals. The former are generally small subhedral and account for 2 to 5% of the modal rock volume. Exsolution patterns are visible in the oxides through a 10 µm electron microprobe beam, exhibiting the separation between magnetite and illmenite although the ulvospinel content for these has not been quantified. The orthopyroxene is sparse and frequently mantled by clinopyroxene and/or partially resorbed in the highly crystalline samples. Crystal sizes are generally small (< 0.5 mm) with a restricted range in composition (En62-65, Fs33-34, and Wo3-4).
3.3.2. Cumulate-Bearing Lavas (Sumb72-74, 80-81 & 91-92)
This group of lavas are summarised in fig. 3.4 (a-c). They comprise the most evolved samples at Sumbing and show evidence for the mixing, or mingling, of two distinct types of magma and/or magma-cumulate interaction (fig. 3.4a and b). Field evidence clearly displays mixing of coarse and fine-grained material and magmas of different compositions, and this is verified in thin-section. These andesites show more phenocryst heterogeneity than rocks from the other groups, primarily due to the variable incorporation of amphibole and biotite.
3.3.2.1. Lava Phenocryst Phases
In common with all of the rocks at Sumbing, plagioclase is the dominant phenocryst phase. These include microphenocrysts and phenocrysts which are mostly less than 1mm and tabular. They all show the textural evidence for disequilibrium as outlined for the pyroclastic deposits, although probably to a lower degree due to the less extensive resorption, melt inclusions and sieve patterns. Furthermore, an anorthite content of between 37 and 59 is far more restricted than for the pyroclastic rocks, with all phenocrysts falling into labradorite and andesine fields (fig.3.4c).