5. Summary of the Fieldwork

Fieldwork undertaken during 1998 (Davis 1999) built on earlier fieldwork (Davis 1983) and systematically sampled the gabbroic units of the Mosedale Series, using amended versions of Hunter and Bowden's maps for reference. Eighteen samples were collected (in 1998) (Table 2), along two approximately NE-SW transects made perpendicular to the strike of the units to enable sampling of each gabbro type. The first was made across the fell top from NGR: NY 342 337 to NGR: NY 352 326. The second was made along the outcropping crags (NGR: NY 351 324 to NGR: NY 355 326), which form the eastern margin of the complex – where the Mosedale Series show the best level of exposure. The lines of transect and the location of sample collections are shown in Figure 2. A brief analysis of the non-gabbroic specimens collected during the field study clearly demonstrated that they were petrologically and geochemically distinct from the gabbro units. PXRF-derived data shows distinctive geochemical clustering of the different rock types sampled, illustrated in Figure 3.

Figure 3

Figure 3: Zr v Y plot for Carrock Fell (corrected PXRF data) to illustrate geochemical clustering of different rock types within the Carrock Fell Complex

5.1. Macroscopic description of the Carrock Fell Gabbro

The high variability in composition and texture within and between the Carrock Fell Gabbro rock units is reflected in a large range of macroscopic features; macroscopic descriptions are of limited diagnostic value. The gabbro is generally phonetic, inequigranular, non-porphyritic, and medium-coarse grained; with a speckled appearance, from white and grey-black, to speckled white-green (due to extensive alteration of primary mafic phases to hornblende and chlorite). The textures range from homogeneous (non-banded), to heterogeneous (non-banded), to heterogeneous cm scale foliated, and dm scale banding with homogeneous bands. Figures 3 and 4 illustrate some well-developed dm scale mafic and felsic banding with cm scale felsic clustering within the mafic horizons.

Figure 4

Figure 4: Photograph of moderately well-developed cm scale felsic and mafic banding (NGR: 347 328)

5.2 Microscopic description of the Carrock Fell Gabbro

The gabbro is highly variable ranging from medium to coarse grained, massive to banded, and leucocratic (with interstitial quartz) to melanocratic. Owing to this high variability, the specimens were grouped according to their modal quartz content, into the following three groups:

  1. <5% free quartz
  2. 5-10% free quartz
  3. >10% free quartz

It was noted that the majority of the gabbros exhibited a constant mineralogy of plagioclase-uralitised augite-Fe oxides ± hornblende, biotite and quartz. Therefore, the focus of the microscope work was centred on distinctive textures and alteration features, which are likely to be more diagnostic of the Carrock Fell Gabbro when observed in an implement thin-section.

5.3 Geochemical signature of the Carrock Fell Gabbro

The results of the ICP analysis are documented in Table 3. Corrected PXRF data for all Carrock Fell rock specimens collected are shown in Table 4. The ICP data was used to establish the geochemical signature of the Carrock Fell Gabbro. Although smaller than the PXRF dataset, the ICP dataset was used for two reasons:

  1. a significant proportion of the gabbro samples were extensively altered to hydrated amphibole and chlorite. The ICP dataset contains the eight least altered gabbros from the sample and is considered to be more representative of the true rock; and
  2. the ICP provides bulk rock analysis and does not have the same acquisitional errors and limitations as PXRF surface analysis as an estimate of bulk composition, and is therefore expected to be more accurate than the PXRF dataset.


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