Download Presentation1__ULTRAMAFICS

ULTRAMAFIC ROCKS
EARTH’S MANTLE IS COMPOSED OF
ULTRAMAFIC ROCKS
• ULTRAMAFICS (ULTRABASIC) ROCKS ARE
IGNEOUS & META-IGNEOUS
 VERY LOW SILICA, LESS THAN 45%
 GENERALLY >18% MgO, HIGH FeO, LOW
POTASSIUM
 >90% MAFIC MINERALS(DARK COLORED
MINERALS, HIGH MAGNESIUM & IRON CONTENT)
INTRUSIVE ULTRAMAFIC ROCKS
IUGS Classification diagram for intrusive ultramafic
rocks based on modal percentages of mafic
minerals. Green area represents typical mantle
peridotite
PERIDOTITE
OSMIUM
HARZBURGITE
LHERZOLITE
PYROPE GARNET GARNET-BEARING
HARZBURGITE
PERIDOTITE – DENSE COARSE-GRAINED
ULTRAMAFIC(IGNEOUS)ROCK CONSISTING OF
OLIVINE & PYROXENE, CONTAINS LESS THAN 45%
SILICA; HIGH IN MAGNESIUM-RICH OLIVINE-- OVER
30% WITH APPRECIABLE IRON
• COMPOSITION OF PERIDOTITES VARY RELATIVE TO
PROPORTIONS OF PYROXENES, CHROMITE,
PLAGIOCLASE, & AMPHIBOLE
• PERIDOTITE NODULES FOUND IN CERTAIN BASALTS
& DIAMOND PIPES(KIMBERLITE) ARE OF SPECIAL
INTEREST TO PALEOGEOLOGISTS: 1) CLUE TO
COMPOSITION OF EARTH’S EARLY MANTLE &
COMPLEX PROCESS INVOLVED
2) P-NODULES PRESERVE ISOTOPE RATIOS OF
OSMIUM (Os) [BLUE-GRAY OR BLUE-BLACK HARD
BRITTLE VERY HEAVY POLYVALENT METALLIC
ELEMENT WITH HIGH MELTING POINT,
TRANSITIONAL METAL IN THE PLATINUM FAMILY]
& OTHER ELEMENTS THAT RECORD THE
PROCESSES OVER 3B YEARS BP.
TYPES OF PERIDOTITE: 1) DUNITE – MORE THAN
90% OLIVINE WITH Mg/Fe RATIO OF ABOUT 9:1
2)WEHRLITE – MOSTLY COMPOSED OF OLIVINE
PLUS CLINOPYROXENE 3) HARZBURGITE –
MOSTLY COMPOSED OF OLIVINE PLUS
ORTHOPYROXENE & LOW PROPORTIONS OF
INTRUSIVE ULTRAMAFICS – FOUND IN LARGE
LAYERED INTRUSIONS WHERE DIFFERENTIATED
ROCK TYPES OFTEN OCCUR IN LAYERS
• ULTRAMAFIC INTRUSIVE INCLUDES DUNITES,
PERIDOTITES, PYROXENITES & TROCTOLITE
(RARE VARIETY HAS GREATER PERCENTAGE OF
CALCIC PLAGIOCLASE); THESE GRADE INTO
ANORTHOSITES
• GABBRO & NORITE OFTEN OCCUR IN UPPER
PORTION OF LAYERED ULTRAMAFIC SEQUENCES;
HORNBLENDITE & RARELY PHLOGOPHITE ARE
ALSO FOUND
VOLCANIC ULTRAMAFIC ROCKS
• RARE OUTSIDE OF ARCHEAN
• ESSENTIALLY RESTRICTED TO NEOPROTEROZOIC
OR EARLIER
• BONINITE LAVAS WITHIN BACK-ARC BASINS
( MANUS TROUGH, NEW GUINEA)
• SUBVOLCANIC ULTRAMAFIC ROCKS & DYKES
ARE RARE
• EXAMPLES OF VOLCANIC ULTRAMAFIC ROCKS:
1. KOMATITE – HOST TO ORE DEPOSIT OF NICKEL
2. PICRITIC BASALT
ULTRAPOTASSIC ULTRAMAFIC ROCKS
• ULTRAPOTASSIC ULTRAMAFIC & MELILITIC
ROCKS – TECHNICALLY CONSIDERED SEPARATE
GROUP BASED ON MELTING CRITERIA
• HIGHLY SILICA-UNDER-SATURATED ROCKS WITH
>18% MgO
• ULTRAPOTASSIC, ULTRAMAFIC IGNEOUS ROCKS
SUCH AS LAMPROPHYRE, LAMPROITE &
KIMBERLITE OCCUR AS DIKES, DIATREMES,
LOPOLITHS OR LACCOLITHS ARE RARELY
INTRUSIONS
• KIMBERLITE & LAMPROITE OCCUR AS VOLCANIC
• DIATREMES & MAARS OCCUR AS SUBVOLCANIC
• VENTS OF PROTEROZOIC LAMPROITE (ARGYLE
DIAMOND MINE), CENOZOIC
LAMPROITE(GAUSSBERG ANTARCTICA),
DEVONIAN LAMPROITE( SCOTLAND)
• KIMBERLITE IN CANADA, SOUTH AFRICA &
RUSSIA
• CARBONATITE ARE RARE HIGH-CARBONATE,
LOW-SILICA IGNEOUS ROCK
METAMORPHIC ULTRAMAFIC ROCKS
• METAMORPHISM OF ULTRAMAFIC ROCKS IN THE
PRESENCE OF WATER AND/OR CARBON DIOXIDE
RESULTS MAIN CLASSES: 1) TALC CARBONATE 2)
SERPENTINITE
• SERPENTINIZATION REACTIONS RESULT TO
CHLORITE-SERPENTINE-AMPHIBOLE TYPES
ASSEMBLAGES WHEN METAMORPHIC FLUIDS IN
CONTACT WITH ULTRAMAFIC ROCKS HAVE LESS
THAN 10% CO2
• TALC CARBONATION REACTIONS RESULTS TO
GRENNSCHIST THRU GRANULITE WHEN
METAMORPHIC FLUIDS HAS LESS THAN 10% CO2
DISTRIBUTION IN SPACE & TIME
• MAJORITY OF ULTRAMAFICS ARE EXPOSED IN
OROGENIC BELTS & PREDOMINATE IN ARCHEAN
& PROTEROZOIC; ULTRAMAFIC MAGMAS IN
PHANEROZOIC ARE RARE & FEW RECOGNIZED
TRUE ULTRAMAFIC LAVAS
MANY SURFACE EXPOSURES OF ULTRAMAFICS
OCCUR IN OPHIOLITE COMPLEXES WHERE DEEP
MANTLE-DERIVED ROCKS HAVE BEEN
OBDUCTED UNTO CONTINENTAL CRUST &
{ISLAND ARCS] ALONG & ABOVE SUBDUCTION
ZONES
PLATE TECTONICS
ULTRAMAFIC ROCKS & REGOLITH
• ALKALI-RICH ULTRAMAFIC ROCKS PROVIDE
EXCELLENT BALANCE NUTRIENTS TO THE SOILS;
PERIDOTITE & SERPENTINITE HAVE HIGH RATIO OF
MAGNESIUM TO CALCIUM BUT DEFICIENT IN
POTASSIUM & PHOSPORUS; CONTAIN TOXIC
AMOUNTS OF CHROMIUM & NICKEL
• ULTRAMAFIC ROCKS CREATE UNIQUE VEGETATION:
ULTRAMAFIC FOREST/VEGETATION IS TYPICALLY
STUNTED OR BARREN & HOME TO ENDEMIC
SPECIES ADAPTED TO METALLIC SOILS
• WEATHERED ULTRAMAFIC ROCKS FORM LATERITIC
NICKEL ORE DEPOSITS