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Key to the Tectonic Rock Cycle Self Test
 Go to Rock Cycle Self-Test Return to PTRC Go to Top of KeyINTRODUCTION:
The earth is an evolutionary rock cycle, and it began with a single parent rock. So, one of the things we are interested in is understanding the sequence of rock development on the earth from the ultramafic parent rock to every other kind of rock. The sequence of fractionation events and processes which first produced one new rock, and then another, and then another, until the complete diversity of rocks found on earth came into existence.
Of course the rocks did not develop as lock-step sequentially as the rock cycle implies, but be that as it may, understanding a logical sequence of fractionating out ever more rock types give us tools for understanding how the earth as a system works.So, to the right is an outline drawing of the tectonic rock cycle labeled with letters and numbers. The letters refer to specific rocks generated at each step in the cycle, the numbers refer to the tectonic processes or Wilson cycle stages by which the rock are generated. Each number and letter on the master page is clickable and will take you to a description/interpretation of that rock in its place in the rock cycle. All these interpretations are in order below. The rocks are deliberately not listed in order of formation in the rock cycle; they are assigned letters just to have some form of identification. But you should think about the formation of the rocks as a sequence of processes, one process leading into another, and then another, and then another, until all the rocks are generated. A - Quartz/Lithic/Feldspathic sediments- Rock Cycle Stage VIb. These are sediments found in the blue field on a QFL. They are generated during a continent-continent collision - Stage H of the Wilson cycle. These rocks tend to be quartz rich because the hinterland rocks usually contain preexisting sedimentary rocks as parents that have already been through one or more cycles of sedimentary rock evolution, and as we know, sediment evolution on a QFL is always toward the Quartz apex. In the Wilson cycle, before the Stage H collision, the to-be hinterland was a divergent continental margin (east side of Stage F) and would therefore have quartz rich sediments already, that would only become more quartz rich in the next cycle. In the tectonic rock cycle these Quartz/ Lithic/ Feldspathic sediments will eventually evolve toward the three attractors on the simple ideal model since that is the fate of all sediments. It will only occur on a tectonically stagle craton, however, such as rock cycle Stage VII. B - Limestone (but including all carbonates, limestones and dolomite) Rock Cycle Stage VII. This is one of the 3 attractors of the sedimentary rock Simple Ideal Model. The limestone is deposited only in warm, clear, shallow water, which requires a place of tectonic stability in the absence of clastic sediments. In the tectonic rock cycle model limestone forms on the first cratons to develop. That these cratons do not show up until later in earth history is shows by the chart on the Compositional Evolution of Earth Rocks. C - Shale - Rock Cycle Stage VII. This is one of the 3 attractors of the sedimentary rock Simple Ideal Model. Shale is generated under many circumstances since it is a natural weathering product of most parent rocks. It commonly shows up in wacke sediments, but commonly appears in pure form in any quiet depositional environment (i.e. generally deep water). In the tectonic rock cycle shale appears on the stable craton along with the other two sedimentary attractors, quartz sandstone and limestone (carbonates). D - Quartz sandstone - Rock Cycle Stage VII. This is one of the 3 attractors of the sedimentary rock Simple Ideal Model. They appear in the yellow field of the QFL diagram. The primary source rock is a felsic igneous rock, such as plagiogranite, alkaligranite, or granodiorite, which have > 20% quartz. In tectonically active regions the quartz is mixed with feldspar and lithics (see QFL), resulting in a less than mature sediment. Like limestones, pure quartz sand requires stringent conditions of tectonic stability in order to form. Therefore, it is associated with stable, peneplained, continental blocks. Thus, in the Wilson cycle this rock will be generated only during Stages A, D, G (west side) and I. On the other hand, if we examine the Compositional Evolution of Rocks chart we observe that quartz sand (also secondary quartzites) appears very early in the earth's history. This might seem incongruous, that a rock requiring great tectonic stability should begin to appear when the earth was young, active, and dominated by volcanic arcs. The answer to this riddle is that many of the batholithic rocks generated during volcanic arc fractionation in the Archaean are plagiogranites, which are rich in quartz. And most of the processes in the Archean were aimed at generating first protocontinents and then microcontinents. Thus, when these volcanic arcs come to the end of their cycle and eroded to a peneplain they left behind small continental blocks scattered across the earth's surface. Each of these would have had a veneer of quartz sandstone. And finally, since continents don't subduct like oceanic lithosphere, these quartz rich sandstones are likely to have been preserved and thus get counted in our sample of rocks. E - Arkose - Rock Cycle Stage VIa. These are sediments found in the tan field on the QFL diagram. Arkose is the weathering product of feldspar rich parent rocks, usually felsic and intermediate igneous rocks, but also schists and gneisses. Typically these are generated when continental basement rock is exposed, either by block faulting (horst and graben) or rifting events, or from the exposure of any batholithic source. In the tectonic rock cycle we would expect arkosic rocks to be generated when the batholith of the micro/proto-continents are exposed, and thus arkoses begin to appear with the first continental cratons. If we examine the Compositional Evolution of Earth's Rocks chart we see that arkosic rocks appear later than lithic or even quartz rich rocks. F - Greenschist Rock Cycle Stage VIII. Barrovian metamorphism; greenschist facies (low grade). Depending on the parent rock these rocks can be many kinds. A shale parent produces slate and phyllite. Quartz sandstone produces quartzite, and limestone (carbonate) produces marble. And of course a mafic parent will produce greenschist the rock. But in the tectonic rock cycle it is sedimentary rocks being metamorphosed so we would expect slates/phyllites, quartzite, and marbles. CONDITIONS OF GENERATION: Barrovian metamorphism may be produced under many tectonic conditions. It occurs around the batholiths in a volcanic arc (Stage E) or during a cordilleran orogeny (Stage G, east side). It may also occur during a collision orogeny, in a foreland continent depressed into the earth by the weight of the overriding hinterland(Stages F and H). In the tectonic rock cycle we presume the metamorphism is the result of a cordilleran or a collision orogeny of some sort. G - Amphibolite Rock Cycle Stage VIII. Barrovian metamorphism; amphibolite facies (medium grade). Depending on the parent rock these rocks can be many kinds. A shale parent produces schist. Quartz sandstone produces quartzite, and limestone (carbonate) produces marble. And of course a mafic parent will produce amphibolite the rock. But in the tectonic rock cycle it is sedimentary rocks being metamorphosed so we would expect amphibolites, quartzite, and marbles. Barrovian metamorphism may be produced under many tectonic conditions; see Greenschist - Rock F for description. H - Granulite Rock Cycle Stage VIII. Barrovian metamorphism; granulite facies (high grade). Depending on the parent rock these rocks can be many kinds. A shale parent produces banded gneiss. Quartz sandstone produces quartzite, and limestone (carbonate) produces marble. And of course a mafic parent will produce granulite, a pyroxene rich rock. But in the tectonic rock cycle it is sedimentary rocks being metamorphosed so we would expect gneisses, quartzite, and marbles. Barrovian metamorphism may be produced under many tectonic conditions; see Greenschist - Rock F for description. I - Migmatite Rock Cycle Stage IX. Magmatite is partially melted rock. In appearance it looks like a gneiss intermixed with patches and splotches of phaneritic (coarse grained) igneous texture. When we talk of fractional melting of an igneous rock this is what the rock is like with part of the original texture remaining, but surrounded by a slush of melt. In the tectonic rock cycle we are presuming a shale parent undergoing Barrovian metamorphism, and since weathering processes responsible for shale production are also fractionating processes, the shale has a composition that will melt to a rock near the bottom of Bowen's Reactions Series and generate a felsic igneous rock. J - Felsic igneous Rock Cycle Stage II. These are the last rocks of the igneous fractionation sequence. They are the alkaligranites and syenites, sweated out of the intermediate parent rocks by fractional melting (which were sweated out of the mafic parent by fractional melting, etc.) Felsic igneous rocks (alkali suite) are generated under at least two tectonic conditions. The first generating mechanism is along subduction zones associated with volcanic arcs and cordilleran orogenies. The second generating mechanism is in association with hot spots coming up under continents (see Wilson Stage B). The mafic mantle magma ponds at the base of the continent, and the heat partially melts the lower portions of the crust. K - Lithic sediments Rock Cycle Stage IV. Lithic sediments are found in the green field on the QFL diagram. Lithic sediments come from the weathering of sourcelands that are likely to weather into nondescript rock fragments, such as aphanitic (fine grained) igneous rocks. Thus, volcanic arcs are likely to generate lots of lithic rich sediments. But the lithic fragments can be lots of things, mafic minerals, other resistant minerals, small fragments of fine grained sedimentary or metamorphic rocks - lots of things. In the tectonic rock cycle lithic sediments come from the weathering of volcanic arcs, and since volcanic arcs are likely to have been the first sourcelands to break above the Archaean sea surface they would be some of the first sediments on the earth. If we examine the Compositional Evolution of Earth's Rocks chart we see that lithic-rich rocks appear very early, and are abundant. L - Blueschist Rock Cycle Stage V. High pressure, low temperature metamorphism in the melange belt (phase diagram). These are the sediments (and any invading igneous intrusions) dragged down into the subduction zone as well as scraped off of and piled up above the subduction zone. They are typically badly faulted, folded and sheared producing the "melange" ("mixture" in French). CONDITIONS OF GENERATION: Blueschist is formed in only one place, subduction zones. Because the sediments begin on the ocean floor they are cold. Rapid subduction takes them into high pressure zones faster than they can heat up, hence the high pressure and low temperature. Blueschist is also typically associated with Barrovian metamorphism where the two together form paired metamorphic belts, a signiture of subduction processes. Blueschist does not last long in geologic history. Most of the blueschist we have is less than 200 million years old. Normally, the blueschist melange gets caught up in other geologic processes where it is altered, initially to greenschist. M - Intermediate igneous ; Rock Cycle Stage II; diorites, granodiorites, plagiogranites. These are the first rocks of the igneous fractionation sequence in a subduction zone. They are the intermediate igneous rocks sweated out of the mafic parent rocks by fractional melting. Intermediate igneous rocks (calcalalkaline suite) are generated along subduction zones associated with volcanic arcs and cordilleran orogenies. Initially it was thought that it was the subducting slab that was fractionally melting, but more recent work indicates it is the mantle material above the plate that is melting. The mechanism seems to be the large volumes of water carried down into the mantle by the subducting plate. The water leaks from the subducting rocks into the overlying mantle rocks where it acts as a flux, promoting the melting. N - Mafic igneous Rock Cycle Stage II. These are the same rocks as ‘O' only now here in the tectonic rock cycle they are entering a subduction zone rather than being generated at a divergent plate boundary. O - Mafic igneous Rock Cycle Stage II. These are the basalts and gabbros (tholeiite suite) of the oceanic lithosphere (layers 2 and 3 of the ophiolite suite.) They are generated at divergent plate boundaries where mantle convection cells rise toward the surface and fractionally melt. P - Anorthosite Rock Cycle Stage I. A rock composed of calcium rich plagioclase (anorthite). Little if any anorthosite is being generated today since we do not find it in the ophiolite suite. But Archaean oceanic lithosphere has lots of anorthosite, interlayered with amphibolite (medium grade metamorphic derived from a pyroxene rich parent.) The anorthosite is derived from fractionation processes of the original ultramafic parent rock at divergent plate boundaries. Apparently, anorthosite is a common constituent of planetary bodies early in their history since anorthosite is one of the major components of the moon's lithosphere. And if a planetary body's evolution is stopped early the anorthosite is preserved, as on the moon. And, during the earth's early history abundant anorthosite was a common product of convection cell processes forming significant proportions of the Archaean oceanic lithosphere. But as time went on the volume of anorthosite being generated declined, probably because the sources in the mantle were depleted and then exhausted. It is hard to know just where all the Ca plagioclase went, but it is quite likely that large amounts of the calcium, released by weathering, has ended up dissolved in the oceans, and then become sequestered (stored away) in carbonate rocks (limestones, CaCO3 and dolomites CaMg(CaCO3)2, whose volume has increased with time (see composition chart). And without the calcium, calcium plagioclase cannot form. Thus calcium plagioclase gets converted into calcium carbonate during sedimentary processes. Q - Ultramafic igneous (residue). Rock Cycle Stage I. Nickel, chromium and olivine rich rocks. We have several kinds of ultramafic rocks, not all the same. All are derived ultimately, however, from the parent rock, the original rock which composed the earth. The ultramafic parent rock contained a little bit of everything, sufficient to supply the materials for all the other rocks on the planet. This Ni/Cr/Olivine rich ultramafic is the first residue derived from the fractionation processes at a divergent plate boundary. This Ni/Cr/Olivine rich ultramafic is a product dominantly of Archaean processes at divergent plate boundaries. As the parent rock migrates toward the surface along convection cells it begins to fractionally melt, sending mafic magma to the surface to form basalts and gabbros (tholeiite suite cross section; definition). The unmelted residue is rich is the most refractory (heat resisting) materials, which happen to be nickel, chromium, and olivine. These become concentrated in layers in the lowermost portions of the Archaean lithosphere. We do not see these rocks forming today, but their formation in the past has been of enormous economic significance. Most of the world's nickel and chromium comes from South Africa, and if these Archaean processes had not generated it, it is fair to say that modern civilization would not be possible since high technology is absolutely dependent on these elements. R - Ultramafic igneous. Rock Cycle Stage III. Dunite and peridotite (olivine and pyroxene rich rocks). Beginning with a mafic parent, this is the unmelted residue of the fractionation processes along a subduction zone. From the mafic source rock (Rock N) an intermediate melt is derived (Rock M) leaving behind the ultramafic residue. See igneous evolution chart. S - Eclogite. Rock Cycle Stage III. High temperature/high pressure metamorphism of an ultramafic parent rock (phase diagram). These are the rocks dragged down into the high P/high T mantle via the subduction zone. T - Ultramafic parent rock (Komatiite). Rock Cycle Stage I - A rock that superficially looks like a basalt or gabbro (pyroxene, Ca plagioclase rich, with olivine) but whose chemistry is high in silica and contains all the other elements necessary to generate all other rocks on the planet, igneous, sedimentary and metamorphic. Technically it is the komatiite suite. This, along with anorthosite, is the rock that composes the moon's lithosphere, and which composed the earth when if first formed. It is possible that little or none of this rock remains on the earth, or if it does exist it is buried deep in the mantle. However, through various igneous and sedimentary fraction processes greater and greater varieties of rock types have been split out from this parent leaving behind greater and greater proportion of fractionated, sterile residues. The initial fractionation processes are associated with rift zones where the compositionally rich magma split into basalts and gabbros at the surface (Rocks O and N), anorthosite (Rock P) and pyroxene rich layers below, and highly refractory components such as nickel, chromium and olivine below that (Rock Q). |