Click here to go to 'Figures/Overheads' section.
Click here to return to course outline.
Click here to return to W.R. Church's home page.
Click here to return to Earth Science course list.
Revision: Plate Tectonic environments; major elements and common trace elements, element compatibility.
Research over the last 20 years has demonstrated that a variety of Plate Tectonic environments can be characterised by the chemistry of the rocks, both volcanic and sedimentary, found in those environments. This has given rise to the idea of chemical fingerprinting as a technique for identifying plate environments in ancient terrains.
Plate Tectonic environments can be classified in a simple way as follows:
| OCEANIC ENVIRONMENTS |
OCEAN
FLOOR
a) Oceanic plates formed by basaltic volcanism at Mid-Ocean ridges. The
volcanic material is known under the acronym 'MORB' or NMORB; e.g. mid-Atlantic
ridge. MORB rocks relatively enriched in alkali elements are known
as EMORB, and varieties between NMORB and EMORB are called TMORB (Transitional
MORB). There is a full spectrum of types between NMORB and EMORB.
b) 1)Oceanic volcanic rock added to the oceanic crust by the intrusion
of basalt related to mantle 'plume' activity, e.g. Hawaiian islands, Iceland,
etc. Acronyms: OIB, OIT, WPT, PMORB (=EMORB).
2) Oceanic small mantle melts of alkalic material related to plume activity.
Acronyms:OIA, WPA; e.g. Hawaii.
3) Archean komatiitic volcanics. Acronyms: PK, BK.
ARCS
c) 1) Very primitive 'Boninitic' island arcs; Bonin Islands;Marianas.
2) Immature oceanic island arcs; low-K tholeiites; pigeonitic series; Acronyms:
LKT, IAT; e.g. Tonga.
3) Mature oceanic island arcs; calcalkaline; hypersthenic series; high-Al
series. Acronyms: CAB (basalt), CAD (dacite), CAR (rhyolite); e.g. Japan,
Aleutians
4) Very mature 'Shoshonitic' island arcs, K2O wt% > Na2O wt%; e.g. Fiji.
| CONTINENTAL ENVIRONMENTS |
a) Continental
rifts; bimodal basalt-rhyolite association; high-Ti basalts, peralkaline
rhyolite. Acronyms: WPB; WPT; WPA; e.g. Afar Triangle, East African Rift
b) Volcanic arcs; Acronyms: CA; e.g. the Andes; NewZealand.
c) Collisional zones. Acronyms: synCOLG; e.g. the Himalayas.
Common
acronyms:
| BK | Basaltic komatiite |
| CA | Calc-alkaline |
| CA | Continental arc |
| CB | Calc-alkali basalt |
| CAB | Calc-alkali basalt |
| CD | Calc-alkali dacite |
| CR | Calc-alkali rhyolite |
| EMORB | Enriched MORB |
| HFT | High Fe tholeiite |
| HMT | HIgh Fe tholeiite |
| IAB | Island arc basalt |
| IAT | Island arc tholeiite |
| LKT | Low-K tholeiite |
| MORB | Mid Ocean ridge basalt |
| NMORB | Normal MORB |
| OFB | Ocean Floor basalt |
| OIA | Oceanic island alkali |
| OIT | Oceanic island tholeiite |
| ORG | Orogenic granite |
| PK | Peridotitic komatiite |
| TA | Tholeiitic andesite |
| TB | Tholeiitic basalt |
| TD | Tholeiitic dacite |
| TMORB | Transitional MORB |
| TR | Tholeiitic rhyolite |
| TH or Thol | Tholeiitic |
| VAB | Volcanic arc basalt |
| VAG + synCOLG | Volcanic arc granite and syn-collisional granite |
| WPA | Within-plate alkali |
| WPB | Within-plate basalt |
| WPG | Within-plate granite |
| WPT | Within-plate tholeiite |
The geochemical fingerprinting is carried out by plotting geochemical data
on variation diagrams, and looking for characteristic inter-element relationships
that characterise each geotectonic environment.
Variation diagrams encompass both major elements and trace elements.
Major elements:
Remember: in bipolar plots of incompatible elements, the primary ratio is given by the slope of the trend line or the ratio of the elements in the most primitive member of the series.
Bipolar plots : 1) SiO2 v total alkalies; 2) FeO v MgO;
Triangular plots : 1) A-F-M; iron enrichment versus increase in FeO/MgO; cumulates versus liquids (use of partition coefficients);
FeO/MgO plots : 1) FeO; SiO2; TiO2; Cr
Cr V TiO2
Boina volcanics of the Afar Rift zone - elements versus residual melt fraction (Note: how the trend lines indicate which elements are behaving as compatible or incompatible elements, and that only K approaches being totally incompatible for most of the fractionation.)
Fingerprinting using trace elements
Crystallization of certain
mineral phases is recognisable in the abundance variation of certain elements
with which the phases have a compatible relationship. The following elements
are particularly valuable in trace element 'fingerprinting'.
Cr - garnet; Y and
Yb - garnet; Ni - olivine; Sc - clinopyroxene; Sr - plagioclase, carbonates
(Celestine - SrSO4); Ti, V, and Co - magnetite; Rb - K-feldspar, biotite;
Ba - Kfeldspar (Celsian, Hyalophane); REE - apatite, zircon, monazite;
Hf - zircon; Ta and Nb - titanite; Zn - spinel, staurolite; Cu - sulphides;
Pt, Pd, Ir - sulphides;
Bipolar plots
Compatible
versus compatible
Ni v Cr
Compatible
v incompatible
Ni v Y;
Incompatible
versus partly incompatible
Rb v Sr
Incompatible versus incompatible
K v Rb; K v Ba; K v
LREE; Nb v Y; Ti v Zr
Titanium versus Zirconium, and Ti-Zr-Sr/2 triangular plot.
Triangular plots : Ti/100-Zr-3*Y; Ti-Zr-Sr/2;
Titanium/100-Zirconium-3Y triangular plot.
Isotopes: 87Sr/86Sr v 143Nd/144Nd
REFERENCES
TRACE
ELEMENT MODELLING
Allegre, C.J. and Minster, J.F. 1978.
Quantitative models of trace element behaviour in magmatic processes. Earth
Planetary Sci. letters, 38, 1-25.
Apted, M.J. and Roy, S.D. 1981. Corrections
to the trace element fractionation equations of Hertogen and Gijbels (1976).
Geochim. Cosmochim.,45, 777-778.
Hanson, G.N. and Langmuir, C. H. 1978.
Modelling of major elements in mantle-melt systems using trace element
approaches. Geochimica et Cosmochimica Acta, 42, 725-741.
Hertogen, J. and Gijbels, R. 1976. Calculation
of trace element fractionation during partial melting. Geochim. Cosmochim.
Acta, 40, 313-322.
Hofmann, A.W. and Hart, S.R. 1978. An
assessment of local and regional isotopic equilibrium in the mantle. Earth
Planetary Sc. Letters, 38, 44-62.
VARIATION DIAGRAMS
Jahn, B-M, Chi-Yu, S., and Rama Murthy,
V., 1974. Trace element geochemistry of Archean volcanic rocks. Geochim.
Cosmochim Acta, 38, 611-627.
Jakes, P. and White, A.J.R. 1972. Major
and trace element abundances in volcanic rocks of orogenic areas. Bull.
Geol. Soc. America 83, 29-40.
Jakes, P. and Gill, J. 1970. Rare earth
elements and the island arc tholeiite series. Earth Planetary Sci. Letters,
9, 17-28.
Nisbett, E.G. and Pearce, J.A. 1977. Clinopyroxene
composition in mafic lavas from different tectonic settings Contr. Mineral.
Petrol. 63, 149-160.
Morrison, M. A., 1978, The use of "immobile"
trace elements to distinguish the palaeotectonic affinities of metabasalts:
applications to the Palaeocene basalts of Mull and Skye, northwest Scotland,
Earth Planet. Sci. Lett., 39, 407-416.
Pearce, J.A. and Cann, J.R. 1973. Tectonic
setting of basic volcanic rocks determined using trace element analyses.
Earth Planetary Sci. Letters, 19, 290-300.
Pearce, J.A. and Norry, M.J. 1979. Petrogenetic
implications of Ti, Zr, Y, and Nb variations in volcanic rocks. Contr.
Mineral. Pet., 69, 33-47.
Smith, R.E. and Smith, S.E. 1976. Comments
on the use of Ti, Zr, Y, Sr, K, P, and Nb in classification of basaltic
magmas. Earth Planetary Sci. Letters, 32, 114- 120.
Taylor, S.R., Capp, A.C., and Graham,
A.L., and Blake, D.H. 1969. Trace element abundances in andesites. II.
Saipan, Bougainville, and Fiji. Contr. Mineral. and Petrol. 23, 1-26.
Thompson, R.N., Morrison, M. A., Mattey,
D.P., Dickin, A.P., and Moorbath, S. 1980. An assessment of the Th-Hf-Ta
diagram as a discrimant for tectonomagmatic classification and in the detection
of crustal contamination of magmas. Earth Planetary Sci. Letters 50, 1-10.
Winchester, J.A. and Floyd, P.A. 1976.
Geochemical magma type discrimination : application to altered and metamorphosed
basic igneous rocks. Earth Planetary Sci. Letters 28, 459-469.
Wood, D. A., 1979, A re-appraisal of the
use of trace elements to classify and discriminate between magma series
erupted in different tectonic settings, Earth Planet. Sci. Letters, 45,
326-336.
PETROGENESIS
Anderson, A.T., Jr., Significance of hornblende
in calc-alkaline andesites and basalts. Am. min., 65, 837-851.
Armstrong, R.L. 1971. Isotopic and chemical
constraints on models of magma genesis in volcanic arcs. Earth Planetary
Sci. Letters 12, 137-142.
Hellman, P.L. and Green, T.H. 1979. The
role of sphene as an accessory phase in the high-pressure partial melting
of hydrous mafic compositions. Earth Planetary Sci. Letters, 42, 191-201.
Miyashiro, A., 1975. Island arc volcanic
rock series : a critical review. Petrologie, 1, 177-187.
Sun, S.S. 1975. Evolution of the mantle
: geochemical evidence form alkali basalts. Geology, 3, 297-302. Discussion
by Gill, J.B. and reply by Sun and Hanson in Geology 1976, 625-631.
Structural Provinces of North America.
Titanium versus Zirconium, and Ti-Zr-Sr/2 triangular plot.
Titanium/100-Zirconium-3Y triangular plot.
RETURN TO:
Click here to return to beginning.
Click here to return to course outline.
