3. Greenstone Origins

The majority of greenstone used for hand axes is a medium- to coarse-grained basic intrusive rock (generally dolerite or sometimes gabbro) that may often have undergone low-grade metamorphism with the development of various secondary minerals. It is these secondary minerals (e.g. chlorite, smectite, epidote, amphibole, pumpellyite) that invariably impart the greenish colour to the rock.

Figure 1

Figure 1: Basic magmatic rocks and their metamorphic equivalents

As seen in Figure 1, there are a number of common basic igneous rocks (based on grain size) and their equivalent progressively altered types (based on increasing metamorphic grade). Thus, many hand axes are, strictly speaking, meta-dolerites, especially if some vestige of the primary igneous assemblage is still retained. If the original pyroxene-plagioclase mineralogy has been completely replaced and the rock has undergone a medium grade of metamorphism (e.g. amphibolite facies) then it will be dominated by the amphibole-andesite assemblage and represent a true amphibolite.

3.1 Mineralogy and texture

Fresh dolerites are predominantly composed of a pyroxene (commonly, augite), plagioclase (labradorite) and iron ore (ilmenite or magnetite), with sometimes either quartz or olivine as important additions. The texture is often ophitic, sub-ophitic or granular.

During low-grade mineralogical transformations, the texture of the rock may also change markedly. Secondary minerals may be observed to replace differentially the original igneous pyroxene-plagioclase, which are left as relicts. In many cases portions of the rock are totally composed of actinolitic amphibole or epidote-chlorite patches. The degree of replacement may appear random, but might relate to various fractures, veins or closeness to the boundary of the dolerite body. If the rock has subsequently suffered deformation, the margins, in particular, will be sheared and exhibit a strong mineral alignment or foliation, taking on the features of a greenschist.

Lithological variation

Dolerites are generally observed as medium to high-level intrusive rocks and form sills or sheets relative to the adjacent country rocks. In some volcanic areas they can be temporally related to associated basic lavas, and represent a deeper part of the magma plumbing system of the volcanic activity.

Figure 2

Figure 2: Lithological variation in a differentiated dolerite sill

One characteristic of many very thick dolerite sills (say, >10m thick) is that they are internally differentiated in terms of their lithology (e.g. Palisades Sill, New Jersey; Walker 1969). An idealised section through a differentiated dolerite sill is shown in Figure 2. Not only does the lithology vary from bottom to top of the sill, but the composition of the primary minerals also varies in a systematic and complementary matter. In general terms the rock types change from largely basic throughout the sill towards minor acid pods and veins towards the top. The chilled margins are distinguished by being fine-grained and essentially basalt in composition relative to the different dolerites in the body of the sill. Such lithological variation is generally explained by in situ crystal fractionation from the base upwards during cooling of the magma. In essence, from a detailed mineralogical investigation of a dolerite hand axe, it might be possible to place where in a thick sill the samples were taken for working.

3.2 Geochemistry

Like many basalts, greenstones (dolerites) also exhibit a range of chemistry within the basic compositional spectrum, largely due to the degree of differentiation, basic magma type and, ultimately, magma generation processes at the mantle source. The first two are important in identifying the chemical nature of the greenstone and its classification, whereas the third becomes important in the geochemical discrimination of a possible regional provenance for the hand axe.

Basalts and dolerites can be either tholeiitic or alkaline in nature with a corresponding difference in chemistry; alkaline basic magmas being generally higher and more variable in incompatible element contents (e.g. Zr, Y, Nb, Ti, P, REE) than tholeiitic. Determination of the magma type of a hand axe would enable identification of an igneous complex from which axes might have been manufactured.

Figure 3

Figure 3: Generalised element group distribution with height in a differentiated sill

As thick dolerites often show variation in lithology, so too can the chemistry vary throughout the sill. As seen in Figure 3, different groups of elements (incompatible versus compatible, and the Fe 'group') behave in different ways with the change from basic to more acidic compositions up the sill. Again element distribution is a consequence of variable crystal fractionation from basic to acidic (Figure 4).

Figure 4

Figure 4: Generalised element trends with progressive fractionation from basic to acidic


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