Plate Tectonic Processes


 
A Plate Tectonic Rock Cycle
 
Descriptions of Processes Operating At Each Stage in the Tectonic Rock Cycle
 
I.  Generation of mafic oceanic crust by fractional melting of ultramafic parent magma at an oceanic divergent plate boundary (rift zone). Stage C and Stage D of Wilson cycle, or at any Divergent Boundary generating Ophiolite Suite.
     
Tectonics: This step begins where the earth's tectonic processes begin, with a mantle convection cell rising toward the surface bringing ultramafic parent magma with it. The Wilson cycle assumes a rifting continent, but rift initiation need not involve a continental mass. Intraoceanic rifting events are possible. The tectonic rock cycle assumes an unspecified intraoceanic rifting of undefined origin.
    
Lithology: The ultramafic parent magma fractionally melts as it approaches the surface creating a mafic melt which forms the oceanic crust (Ophiolite Layers 2 And 3), leaving behind in the mantle an ultramafic residue (ophiolite layer 4).

Although most rocks in the tectonic rock cycle are straight forward, the ultramafics present possible confusion since they appear more than once, in different guises. The first ultramafic is designated the parent. It is the compositionally rich magma which fractionates at rift zones. The second ultramafic in the tectonic rock cycle is the sterile fractionated residue generated by fractionation of Ophiolite Layers 1 And 2 at The Subduction Zone

In the ARCHAEAN, the earliest and oldest phase of earth history, the mafic parent was called the Komatiite Suite, and it generated an oceanic lithosphere quite different from that generated today. 

II.  Generation of intermediate/felsic igneous rocks by fractional melting in a subduction zone to form diorite / andesite (IIA of the Tectonic Rock Cycle) and eventually Granites (most likely plagiogranite)/ rhyolite. These will form in either Stage E of the Wilson Cycle, or Stage G or any comparable stage where there is a subduction zone.

Tectonics: this phase of the tectonic rock Cycle has shifted from a Divergent Plate Boundary where oceanic lithosphere is generated to a Convergent Plate Boundary where oceanic lithosphere is destroyed. Subduction zones may form either intraoceanically (Wilson Stage E), or along continental edges (cordilleran type; Wilson Stage G). Because cordilleran orogenies have igneous and metamorphic processes similar to volcanic arcs, and because most cordilleran orogenies end in a Collision Orogeny, which is a later part of the tectonic rock cycle, they are not explicitly discussed here.

Lithology:  The Ophiolite Suite gradually heats as it descends because of the geothermal gradient and friction of subduction. The descending slab also carries a lot of sea water with it and at about 120 km depth, the water and heat lead to Fractional Melting of the mantle material just above the subducting slab. As heating progresses only the lower temperature Phases (lower on Bowen's Reaction Series) in the rock melt to produce magmas of intermediate composition. Since these are fluid and hot they rise up through the crust to eventually emplace and solidify as intermediate rocks (e.g. DioritesGranodiorites, etc). The second fraction is the unmelted residue with a composition more mafic/Ultramafic than the original rock. That is, its composition is higher in Bowen's Reaction Series than the original rock. 
    
The initial melt may be mafic (tholeiitic suite) (the reason Gabbro/Basalt appears at both Steps I and IIA), but evolves through time (and space) to intermediate (Diorite/Andesite) and felsic (Plagiogranite/Rhyolite) rocks. Magma rising toward the surface creates a Volcanic Arc. Emplacement of batholiths creates new continental crust.
    
If a subduction zone is active long enough a considerable mass of intermediate and felsic batholiths (calcalkaline and alkaline suites) can accumulate. If there are enough batholiths they can form a Protocontinent, the small nuclear core of what could become a large continent through repeated Wilson cycles occurring along its margins. In the Archaean, the earliest stage in the earth's history, all the continents began as volcanic arcs that either collided together to form small protocontinental blocks, or as volcanic arcs that were active long enough to create large enough masses of batholiths to become protocontinents (Cross Section Series).

III. Generation of high P/high T ecolgite by descent of unmelted oceanic lithosphere (ultramafic residue) into mantle along subducting plate, accompanied by high P/high T (Eclogite) metamorphism.Stage E (and in more complicated tectonic rock cycles, Stage G) of the Wilson cycle.

Tectonics: a process coupled with the fractional melting taking place above the subduction zone (Step II). As the oceanic slab descends and fractionally melts, the unmelted ultramafic residue remains in the mantle or continues to descend with the subducting oceanic lithosphere. 

Lithology: the oceanic lithosphere continuing to descend into the mantle is fractionated twice; once at the oceanic rift center, and a second time along the subduction zone. It is a sterile residue (as opposed to a parent) ultramafic rock. There is nothing more to fractionate from it. At high temperatures and pressures inside the mantle, it metamorphoses to Eclogite, the first metamorphic rock in the tectonic rock Cycle.
 
IV.  Generation of lithic rich sediments by weathering of a volcanic arcStage E of the Wilson Cycle.

Tectonics: an outcome of the continuing process of subduction in Steps II and III.

Lithology: once the volcanoes rise above the sea surface active weathering begins. Because mostly volcanic rocks are weathering the sediments are lithic-rich. On deeper weathering, down into coarser igneous rocks (or shallower porphorys) feldspars may be added (Follow Red Arrows On QFL Diagram). If plagiogranites or granodiorites are present more or less amounts of Quartz will be present. The result is lithic-rich to feldspar-rich sediments deposited in submarine fans surrounding the volcanic arc, the first sedimentary rocks of the tectonic rock cycle
 
V. Generation of high P/low T Blueschist melange along a subduction zone. Stage E (and in more complicated tectonic rock cycles, F) of the Wilson cycle.

Tectonics: a continuing process of the subduction zone in Steps II, III and IV. Sediment weathering from the volcano pours into the trench where it is scraped off and upward to form the melange belt of the Outer Arc Ridge.

Lithology: on the backarc side the lithic sediment enters geologic dormancy for some unspecified time. On the forearc side the sediment enters a trench. Some sediment is scraped off and thrust upward forming an outer arc ridge. The remainder of the sediment is subducted. Because the sediment goes down quickly along the subduction zone, pressure on the sediment rises rapidly while temperature rises slowly; this is Blueschist metamorphism. 
 
VIa. Generation of arkosic sediments on a Micro(proto)- continent. 
 
Arkose is the second sedimentary rock generated in the tectonic rock cycle (QFL)
     
Tectonics: in an evolutionary tectonic rock cycle, Step VIa is formation of a proto(micro)continent. If an intraoceanic subduction zone is active long enough, a large volume of intermediate and felsic batholithic rocks are emplaced forming a low density plug in the lithosphere (Detail). At the end of the volcanic cycle, active mountain building ceases and erosion removes the volcanoes. Eventually the batholiths are exposed and eroded to a peneplain, leaving them floating isostatically as a micro(proto) continent just above sea level.(Developmental Series).

Lithology: in Step VIa the weathering and eroding sediments evolve through a sequence of maturation. At Stage IV sediments are lithic-rich, but as intermediate and felsic batholiths are exposed in Step VI feldspar-rich Arkosic sediments result (QFL; Follow Red Arrows).  Arkose is the second of the major QFL sedimentary rocks to appear in the model. In Step VII they complete their weathering Sequence.

VIb. Generation of Quartz/- lithic/feldspar sediments from a major mountain building. 
    
Tectonics: Step VIb is a collision style orogeny (Wilson Stage F, or Wilson H), or a cordilleran orogeny ( Stage G). Subduction zones create remnant ocean basins, which will eventually close causing a collision between arcs, or arcs and continents, or Continents And Continents. The collision style orogenies build large thrust belt mountains in the hinterland, and foreland basins in the foreland. Such collisions bring mountain building to an end since the remnant ocean is closed by these episodes.

Lithology: in Step VIb the thrust belt mountain contains a great diversity of rocks, including volcanic, sedimentary, metamorphic, and uplifted intermediate and felsic basement which form the core of the continents. As these weather and erode a mixed Quartz/feldspar/lithic sediment results (QFL, Blue Field). These sediments are characteristically Quartz rich (>50%) because the sedimentary rocks weathering from the hinterland have already been through at least one cycle of weathering and thus have a high Quartz content. 
    
As long as the mountain remains high and weathering is rapid and incomplete the sediments remain immature to submature. As the mountain erodes, however, these sediments evolve through maturation, which is complete in Step VII. 

VII. Generation of Quartz/Shale/Limestone on a peneplaned (micro/proto) continent. 
    
Tectonics: the (micro/proto) continent is stable, weathered and eroded to a peneplain. Such a situation shows up three times in the Wilson cycleStage AStage G (west side), and Stage IStable continental blocks are not common on the earth today; we seem to be in a time of intense geological activity across most of the earth. However, western Australia and central North America (the plains) are examples. Even these do not appear as stable as times in the past when very mature Quartz Sandstones were deposited across wide areas.
 
Lithology: the arkose from Step VIa, OR the Quartz/lithic/feldspar sediment from Step VIb are completely weathered. The feldspars and lithics have gone to Shale and dissolved salts. The Carbonate salts precipitate out either chemically or biochemically forming Limestone and Dolomite. The Quartz remains as an unweathered residue.
    
With Quartz sand, Shale, and Carbonate we have the final end-member products, the three attractors, of weathering/sorting/deposition process that has gone to absolute completion (Tectonic QFL Diagram).

VIII. Barrovian metamorphism 
    
Tectonics: as long as the Wilson cycle operates, virtually any floating lithospheric block eventually is impacted by an orogeny, either with a subduction zone along its coast, or by colliding with another floating block. The Wilson cycle model has two collision Events; volcanic arc/continent (Stage F), and continent/continent (Stage H), but arc/arc, and arc/microcontinent collisions are just as possible.  Subduction zones originate in many situations. Stage E of the Wilson cycle is intraoceanic, and Stage G is cordilleran. 

Lithology: many rocks are generated during one of these orogenies however, Barrovian metamorphism acting on a sedimentary (primarily Shale) parent is dominant. Rocks other than Shale are not likely to undergo additional, major transformation or remobilization during Barrovian metamorphism. For example, the long term fate of the plutonic rocks (e.g. Diorites and Granites), now part of a continent, is most likely metamorphism to Gneiss, which is a textural change but not a significant compositional (mineralogic) change. 
    
Therefore, the tectonic rock cycle focuses on Barrovian metamorphism of the sedimentary record, and within that record, Shales. Shales are chosen not only because they are volumetrically more abundant than other sedimentary rocks, but also because they have the most interesting and diagnostic transformations during metamorphism. Quartzite and marble formed under Barrovian metamorphic conditions are usually associated with slates, Phyllites, schists, and Gneisses (and their index minerals), the metamorphic indicators of most interest. 
   
During a collision/cordilleran orogeny sediments are subject to increasing temperatures and pressures, either because they are invaded by batholiths, or because a hinterland plate shoves them deep into the earth (example; Continent-Continent Collision). The result is the Barrovian transformations with increasing grade: Shale to slate to Phyllite to schist to Gneiss
 
IX.  Migmitization and the Creation of Felsic Magmas. 
    
If Barrovian metamorphism becomes extreme enough, beyond the Gneiss (granulite) range, the rock begins to melt and form magma.  
    
Migmatite is a partially melted rock. That is, a rock undergoing fractional melting. As Bowen's Reaction Series predicts, the fraction that will melt first will be the fraction lower on the reaction series. If the melt fraction were separated from the unmelted residue fraction then a magma much lower on the reaction series would form and would crystallize into a felsic rock of some type.
    
Fractionation takes place at virtually every step of the rock cycle. The mafic parent magma undergoes fractionation to form an intermediate rock. An intermediate rock can undergo fractionation to form a felsic rock.  Fractionation occurs even among sedimentary processes. The processes of weathering tend to separate out the elements from an igneous rock, concentrating elements lower in the reaction series in clastic sediments. Even when beginning with a Shale, and metamorphosing and melting it completely, a felsic igneous rock is produced. 

The Earth is not just a rock Cycle. It is an evolutionary rock Cycle.

Contributed by Lynn Fichter 

Friday, November 14, 2014
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